Covering Blue Foam

[Piecost]

This thread covers experiments in covering blue foam to provide a resiliant, smooth surface. Painting straight onto foam is standard practice in the Uk and is the lightest option. But it lacks the hard looking surface to represent metal skins and is easily damaged.

Tje following experiments are based on my interpretation of the Czech model forum using Google translate. In particular; the following threads

https://www.minimakety.cz/forum/viewtopic.php?f=6&t=414&start=36

Less useful thread
https://www.minimakety.cz/forum/viewtopic.php?f=6&t=695

Any help in understanding this information will be welcome.

Furthermore, the following post was made in the old incarnation of the HPA forum



I chose to experiment using the same tissue that I had bought for testing on open frameworks.

I had obtained 3 weights of Conservation Kozo Shika; 4gsm, 6gsm & 9gsm. I did not purchase the 11gsm since it is similar in weight to the light aeromodelling tissues.

I had experimented with sealing these over an open frame in the following thread:
https://hpa.aeromodelling.gr/index.php?topic=419.0

For this test I made test samples of Floormate 200 blue foam, hotwired to a couple of mm thick and measuring 100mm by 100mm square. The hotwire fluff was removed using parcel tape and the surface sanded with an 800 grit sponge.

I weighed the samples at each stage so as to be able to quantify the increase in mass (in gsm) due to; the tissue, the wallpaper paste and to the Sand'N'Seal (S'N'S). The end result was the total mass per area of the surface treatment.

The tests were not repeated  and are subject to inacuracy so need to be taken with some caution.

The following surface treatments were tested:

Conservation Kozo Shika (CKS) 4gsm  with multiple coats of Sand'N'Seal  (S'N'S)
Conservation Kozo Shika (CKS) 6gsm with multiple coats of Sand'N'Seal  (S'N'S)
Conservation Kozo Shika  (CKS) 9gsm with multiple coats of Sand'N'Seal  (S'N'S)
Lense Tissue 9gsm with multiple coats of Sand'N'Seal (S'N'S)
No tissue with with multiple coats of Sand'N'Seal (S'N'S)
No tissue with multiple coats of Pledge Multi-Surface (acrylic) wax

Each tissue was stuck down with Metylan wallpaper paste (mixed from a powder in strongest ratio shown on the packaging)

The surface of the foam was wetted with the paste and the tissue laid down. More paste was brushed on from the middle to smooth the tissue down without air bubbles. The tissue was wrapped around the edges of the samples.

CKS 4gsm creased due to repositioning during covering and some holes apeared. It was very difficult to apply. Once it was laid down, it could not be repositioned without damage. I would not be confident covering a model with this.

CKS 6gsm was easier to apply intact but a perforation along top edge later occured due to sanding

CKS 9gsm was easy to apply but also sustained edge perforations following sanding

The Lense tissue 9gsm  was very easy to apply and did not perforate at any time.

[Piecost]

Continued...

Multiple coats of S'N'S were applied, each sanded with an 800 grit sanding sponge. An oblique light was used to show the lowspots at the pores of the tissue. Care was taken to only remove the high spots of filler. The low spots were progressively filled with each coat and the overall thickness increase was minimised

It took about 3 or 4 coats to achieve a generally uniform surface. I did not seek perfection.

Half the sample was painted with 50% thinned Tamiya NATO black to visualise the surface quality. Half of the sample was then further painted with Vellejo Metal Colour Aluminium; partially over the unpainted and black painted areas. This was to see the impact of the black undercoat. This is the finish I intend to apply to a forthcomming model. I am having trouble airbrushing the ali colour and will post a photo of the samples after giving them a second coat.

If I were seeking a better finish, with consequent weight gain, I would sand off most of the black paint to show the low spots and repeat the filling and sanding

I attach a table and a graph of the gramms per square meter of each finish. The increase in mass due to the tissue and wallpaper paste is given in the top row, labelled as 0 sanded coats (of S'N'S).



I was shocked that the mass of the wallpaper paste was between 1.6 and 3.4 times the mass of the tissue! The tissue varied between 4gsm and 9gsm, but the paste varied between 6gsm and 30gsm! The mass of the tissue was not important relative to the mass of the paste. I only hope that more paste means that less S'N'S is needed.

The lightest tissue certainly used less paste, but there was not a stronge correlation between the different masses of the CKS and the mass of paste used. It was probably due to variation in my technique.

The next table presents the mass of the treatments for each coat of S'N'S. The top row (coat 0) gives the mass of the tissue and wallpaper paste only. each subsequent row includes an additional coat of S'N'S.


id=7825]Screenshot_20260925_085223_Files by Google (1).jpg[/attach]
[INSERT TABLE OF MASS PER COAT]

The final table presents the.cumulatibe mass of each coat:

id=7827]Screenshot_20260925_085435_Files by Google (1).jpg[/attach]

This is plotted with mass of the surface treatment on the vertical axis and number of coats on the horizontal axis.

 id=7828]Screenshot_20260925_085703_Files by Google (1).jpg[/attach]

Conclusions

Lense tissue was applied without damage. All threee CKS tissues had holes apear during application of sanding. These regions were vulnerable to dent damage and exhibited a rougher texture after painting. Where the tissue was intact; the surface texture was similar across all samples, with and without tissue.

The lense tissue 9gsm required 12gsm more wall paper paste than the CKS 9gsm. The paste was over 3 times the mass of the tissue!

There is some correlation of mass of wallpaper paste to different masses of CKS. The lightest definately used.less, but the 6gsm and 9gsm used the same. Perhaps this is consistent with the medium and heavier CKS looking and feeling similar, whereas the 4gsm feels more gossimer.

The difference in mass of wallpaper paste made more impact than the mass of the tissue itself! The (dryed) wallpaper paste was between 1.6 and 3.4 times the mass of the tissue.

This significant and supprising result is counteracted by the mass of the S'N'S required to achieve a smooth surface.

The lense tissue exhibited a uniform weight gain with each coat of S'N'S. It only required three coats to achieve a smooth surface.

The more porous CKS, of all three weights, required four, with the associated higher weight gain. However; the number of coats was not decisive.

What was decisive was that all three CKS tissues took a really heavy initial coat of S'N'S. This made the 6gsm and 9gsm CKS similar weights to the Lense tissue; after the first coat.

Susequent coats on all tissues then gave a similar weight gain  to that of the lense tissue.

In the end: the heaviest finish was the CKS 9gsm, then the CKS 6gsm, then the lense tissue. See the yellow cells in the table for the final weights.

I was shocked at how heavy the tissue finishes are; varying between 42 gsm and 68 gsm. This is between double and tripple sealed light tissue over an open frame (around 20gsm).

A more aggresive sanding is would yeild small weight savings. But, most of the weight is due to the wallpaper paste. I think that it helps to fill the pores and any savings on paste would be counteracted by increased mass in S'N'S.

I plan to use one of these techniques on
my next model; a 1:20 rubber scale Pfalz DIIIa. This is the Alfa model short kit, but.I am employing a blue foam fuselage rather than the nice vacuum formed depron? Kit part.

I want to try a tissue finish on the blue foam fuselage, even though I strongly suspect that it will be too heavy. I note that the alfa model kit:

https://www.alfamodel.cz/index.php?_RQhow=listdetail&_RQwhat=shortkit&_RQid=0404&_RQid_2=0222%2FB&_RQlang=eng&_RQstate=usa

Seems to paint directly ontop the foam fuselage (without tissue).

So following the results of the test:

I discount the two lightest finishes, which lack tissue, as they are not resistant to dents.

All three CKS experienced perforation damage, which would nessitate rework on the model. Furthermore; the 6gsm and 9gsm showed no weight saving relative to the lense tissue.

So; I will use the lense tissue. I have tried it on a complex double curvature sample and it conforms well. I noted that the lense tissue warped the foam due to tissue shrinkage. This is a worry, but I plan to cover the fuselage before splitting the halves and hollowing them.

[Piecost]

Lense tissue sucessfully applied over a complex shape.

[OZPAF]

Very impressive research Piecost - thanks for doing this.

I'm amazed at the weight added by the wallpaper paste. I seem to remember PVA being used to seal the surface of foam ARF's

Perhaps this approach may work - an alternative to using a tissue base for paint

Finishing. Flat finish is easiest and lightest--simply spray acrylic paint directly to the finely sanded and prepped foam. A glossy finish can be achieved by prepping the surface--In the words of Doug Wilkey: A neat finishing trick for foam is to mix white glue, Knox gelatin in water and talcum powder. Dissolve the Knox gelatin in water according to the instructions then add equal amounts of white glue, and then add unscented talcum powder. This partially fills the grain and also gives a hard shell to the foam. When the water evaporates from the solution, very little weight is gained. It becomes a very light weight sanding sealer. You can then airbrush a light coat of any of the acrylic hobby paints to get your final colors. Poly S is my favorite when thinned with Rubbing alchohol. Add a drop of liquid detergent to a bottle of mix to keep it from beading up.

John

[Piecost]

Thanks for the informatoon John. It could be worth investigating. I had tried using lightweight filler, but not systematically.

I attach a photo of the test pieces.


It does not show the detailed surface finishes well.

The black paint showed clearly the surface imperfections. For example, the delicate pledge sample sustained a finger nail mark.

The black paint undercoat under the aluminium paint certainly helped coverage

I had great difficulty in obtaining an even coverage of the aluminium paint. It did not airbrush well, even when thinned 50% with Tamiya thinners. The finish is not even and is blotchy. More work is needed before using on a model.

I used yellow Frog Tape to mask the samples. This proved a significant test of the peel strength of the finish. The smooth surface increased the adhesion strength of the tape relative to a sanded foam surface. It damaged some of the CKS 4gsm, removing some if the tissue on the unpainted part (this can be seen just below the name label). On the CKS 6gsm I tried some Isopropyle Alchohol to soften the adhesion, but it damaged some of the black paint. I might try some (not Frog brand) pink masking tape in future, this having less stickiness.

The samples where the tissue had holes showed as rough areas of the painted surface. I consider those tissues which sustained holes as having failed the suitability test.

The S'N'S and Pledge had the best painted surfaces. The Lense tissue was very close behind.

I have some further tests in mind to reduce the significant mass increase due to the wallpaper paste.

[Piecost]

Follow-up test

From the initial test; the supprising points were:

1. The significant mass of the Metylan wallpaper paste used to adhere the tissue
2. The significant weight gain on the initial coat of S'N'S filler to fill the pores if the tissue

I made the following test pieces:

A. Lense tissue 9gsm adhered with a minimum of Metylan wall paper paste. To minimise the weight gain from the paste. Nothing to do with this particular test, but I added a drop of PVA to the Metylan, as per Czech practice.

I applied the paste sparingly with a soft brush, laid the tissue down and did not brush more paste on top to wet the tissue

B. Lense tissue 9gsm adhered with S'N'S rather than wallpaper taste. To test if the wallpaper paste is heavier than the S'N'S in filling the pores.

With both samples I tried to be more aggressive with sanding, with mixed results.




The initial mass of the sample with a minimum paste, before applying S'N'S was more than halved (from 39gsm down to 16gsm).

However, the largest weight gain (of 28gsm) yet experienced on applying the first coat of S'N'S was experienced. I also tore the tissue on sanding and had to patch it. I don't think that the repair added much weight and was inconspicuous after further filling and sanding.

This sample needed 4 coats. This was not important since I was sanding more thoroughly; the weight gain was insignificant. This sample finished at 45gsm, 10gsm less than my previous use of lense tissue.

This sample was also damaged upon removal of yellow Frog tape. So, it failed the test.

The sample which used the S'N'S rather than Metylan wallpaper paste, to adhere the tissue did not save any weight relative to the first test with lense tissue. Infact;  it was heavier than the new test using the paste sparingly.

I attribute this to weight being saved by eliminating the paste was exactly compensated by the increased S'N'S needed to fill the pores.

It did not tear during sanding and was not damaged when yellow Frog tape was peeled off. It passed the test.

Conclusions

Agressively saving weight on wallpaper paste is probably not a good idea. The adhesion was lowered so that damage occurred during sanding and removal of masking tape. Perhaps, the foam surface was contaminated with sanding dust? Or, that adding the PVA was a bad idea and reduced adhesion? It was not good scientific practice to add the PVA; since I can't be sure what caused the reduced adhesion.

It would be prudent, in future,to remove remove and dust with a damp cloth or parcel tape.

I recommend to still wet the tissue after laying it down, but remove excess with a paper towel.

The S'N'S seems to be a viable replacement to wallpaper paste. The sanding and masking tape peel tests were passed. But, the large saving in weight on paste is counteracted by increased weight in S'N'S.

A large weight gain upon applying the initial coat of S'N'S cannot be avoided as it is nessesary to fill the pores of the tissue.

More thorough sanding prevents signifiant weight gain when applying subsequent coats of S'N'N to completely fill the pores.

Tissue patch repairs and lap joints are inconspictuous.

[Piecost]

It bothered me that the impact of the drop of PVA in the Metylan could be degrading the adhesion of the tissue and that this was compromising the previous results.

So, I performed one last test. This used the same lense tissue applied with Metylan. The foam was sanded then parcel tape used to remove any dust. A more liberal amount of paste was applied prior to the tissue and a layer applied ontop the tissue. Care was taken to only apply enough to wet the tissue and not to pool ontop of it.

3 coats of S'N'S,.sanded, resulted in a final mass of 40gsm. This is slightly lower than my best result with this tissue. It is demonstating the scatter in the method.

There was not damage incurred during sanding, but the yellow Frog Tape removal caused two blisters, as shown in the photo.

I conclude that the addition of PVA reduces the peel strength and should not be continued. This contrary to my understanding of the Czech forum, but I am not confident in the exact type of glue used as an additive.

A lower tac masking tape should be used with this surface. The yellow Frog Tape stuck so well that itnexerted considerable force when removed.

[lincoln]

Depending on how thick the foam is, I wonder if it wouldn't be lighter to use a foam with a higher compressive strength and skip the paper. For instance, according to some figures I've seen, Floormate 700 is only 19% heavier than 200. I've messed around with Highload 60, which, if my figures are right, is supposed to have about twice the compressive strength of Floormate 200. It can be wet sanded to a very smooth finish and isn't much heavier. There is also Highload 100. Maybe those are very similar to Floormate xxx.

When hotwiring, if you can regulate the speed of the cut consistently enough, the wire can cut without actually touching the foam. This eliminates fuzz and leaves a relatively smooth surface.

You might try applying the tissue to the foam with Titebond thinned with a little water. I THINK I was using Titebond III when applying kraft paper to repair foam on a Radian RC glider. It might have been another flavor. The result was smooth and strong, but of course it was far heavier than what you want. I wonder what the result with tracing paper would be.

[Piecost]

Thanks licoln, I am pleased that poeple are starting to post ideas, for future reference. I will be sticking with Floormate 200 as being the lightest extruded polystyrene that I could aquire. I have a lifetimes supply, also.

Tim

[Piecost]

lincoln sorry for getting your name wrong. It must have been about 20 years ago thay I bought a whole 10 sheet pack of Floormate. It is a shame that it is not easier to obtain single sheets. A while ago I obtained some different types from a, now defunct, art supplier. I was comparing different foam stiffnesses for use on flapped F1N glider wings.

Tim

[lincoln]

Mangling my name doesn't bother me a bit, as long as I can tell you mean me. I once walked to the nearby insulation warehouse and came back with a single piece of Highload foam, 2" X 24" X 96". That's a little awkward to carry by hand, though.

Original message, before I saw your response, follows.

If my numbers are right, using 700* instead of 200, 2mm thick, only costs you 12 gsm. Maybe a little more when hot wired. If it wet sands like Highload 60, it will probably be smooth enough to look like metal when painted. Or, at least, my foam looked VERY smooth by the time I got to, I think, 600 grit. For all I know, maybe that would also be true for Floormate 200. It would be much less work than adding tissue, too. One minor problem is that the surface becomes so smooth that it's hard to see the shape anymore!

If using up the 200 becomes a problem, use it for wing cores or insulating your house. ;-p Hmm.. that reminds me, winter is coming and maybe I should add a bit more insulation rather than playing on the internet. And do approximately a gazillion other things outside that require reasonable temperatures. There is no end to tasks I really should be doing. (pauses to scream) OTOH, I can only do one at a time. Is infinity minus one significantly less than infinity?

*If I remember, these numbers refer to the compressive strength rating in N/mm2 times 1,000.

[Piecost]

I have started experimenting with a technique developed by Tonda Alfrey, as described in the Czech aeromodelling forum;

https://www.minimakety.cz/forum/viewtopic.php?f=6&t=414&start=36

This is to pre-seal the tissue before applying to the foam surface.

I have followed the following process in an attempt to replicate the process: as follows:
1. Lay the tissue on a shiny surface, such as wallpaper backing paper
2. Sponge on a thinned metal lacqure to bind the fibres together to withstand item 3
3. Use a foam roller to apply an acrylic floor polish;.a Czech brand called "Balakryl"
4. When thoroughly dry; peel off the backing an use Metylan wallpaper paste to adhere to the foam

The smooth backing causes the coated tissue to form a smooth, glossy texture.

My initial test tried 4 types backing; 2 types of silicon infused greaseproof paper, shiny laminating wallet and glass, with release wax and PVA release agent. Three of these worked; except the laminating wallet, which stuck fast. The greaseproof paper released well, but its surface was not smooth enough.

I used Conservation Kozo Shika (CKS) of varying weights.

I persisted with using glass as being easier to manage than greaseproof papaer.

I initially applied five coats of wax, then PVA release agent. The tissue was laid flat, not taped down or it will wrinkle.



I had aquired Zaponlack Metalls hutzlack (metal lacqur). It smelled of cellulose so I tried to obtain cellulose thinners from a motor factors. I was only able to get a universal thinner, which did seem to work. I mixed in 1:1 ratio by volume and sponged it onto the tissue to fully wet it,  Working from the middle outwards to stick the tissue to the backing and to avoid creases and without tearing the fibres apart.

I was not very careful in the amount applied. It smelt strongly and hopefully much added weight evaporated.

When dry, pored on some Pledge Multi-Surface (acrylic) wax, and used a foam roller to spread it out. Again, I was not careful in the amount applied when dry; I peeled up a corner whichn showed many pin holes. A further coat of Pledge fully sealed the surface.
It was left to dry for two days and then carefully pealed off.



The side of the tissue facing the backing was glossy and smooth.

It produced a smooth shiny surface on the tissue, but the PVA was stuck to the surface of the tissue, adding weight and possibly comproming application of paint

Unfortunately, the use of Pledge did not give the flexible, rubbery, surface given by the use of the mysterious Czech Balakryl substance. The coated tissue was stiff and would not conform to the slightestdouble curvature.



I sparingly applied Metylan wallpaper paste to some foam, applied the coated tissue, shiny side up and smoothed it down.

The pre-coated CKS 6gsm tissue onto foam weighed 33 gsm. The surface was not as smooth due to imperfect application of PVA

The CKS 6gsm with 2 coats of pledge weighed 33 gsm. This is lighter than gluing the tissue to the foam and then sealing and sanding. Also, a lot less work.

Very careful to use the absolute minimum of wallpaper paste to stick to the foam.

I have found that the PVA release agent is not needed, waxing the glass adiquate. I am preparing a sample without it.

The coated tissue was stiff and unlike Tonda's report of using. I have ordered somw different acrylic varnish to try and reproduce the flexibility.

Any thought on a suitable varnish to replicate the characteristics of the Czech "Balakryl"

[RolandD6]

I have started experimenting with a technique developed by Tonda Alfrey, as described in the Czech aeromodelling forum;

https://www.minimakety.cz/forum/viewtopic.php?f=6&t=414&start=36

This is to pre-seal the tissue before applying to the foam surface.

I have followed the following process in an attempt to replicate the process: as follows:
1. Lay the tissue on a shiny surface, such as wallpaper backing paper
2. Sponge on a thinned metal lacqure to bind the fibres together to withstand item 3
3. Use a foam roller to apply an acrylic floor polish;.a Czech brand called "Balakryl"
4. When thoroughly dry; peel off the backing an use Metylan wallpaper paste to adhere to the foam.

...

A few years back I experimented like your current efforts. I tried glass and waxed paper and so on but got the best results using smooth HDPE sheet for the backing surface. I recorded lab notes at the time but they are not at hand at the moment, because I am writing this in hospital.

Like you I tried Pledge but found it dried hard and crumbled. The composition of the Pledge I used may not be exactly the same as what you tried. I found a locally made acrylic floor polish worked the best. I was able to pre-color the Kozo tissue with Jacquard dye which contains very thin acrylic resin and pigment. Food dye does not work because it is water sensitive even when mixed with acrylic floor polish. I did not try Rit or Dylon fabric dyes primarily because of cost and storage of large volumes of pre-mixed solutions. The may work fine.

I cannot recall finished tissue weights but I know for certain they were nothing like 33 gsm that you are mentioning.

I should be home tomorrow afternoon and try to find my notes.

Paul

Addendum: I have a peanut model that has been hanging around for a few years in a safety box. I was going to digital print 9 gsm Kozo I may change tactics after reading my notes.

[Piecost]

Hi Paul, I hope that your hospital visit was sucessful.

I am delighted that you have posted. As I stated before; the Czechs have done a lot of work, but it can be challenging to undertand what precise materials they used and to be able to obtain equivalents in the Uk. It would be good to have an English language repository of such experiments. So, I look forward to any further information and/or photos that you are able to share.

Can you explain a bit more about the type of plastic that you used? Did you need any kind of release agent? I was very disapointed that the laminating wallet stuck to the treated tissue, since it had the shiniest surface of any plastic I had seen. Perhaps, I will try waxing it. But, ideally, I would like to avoid that step. I also have some 0.003" thick mylar, which could be worth a try.

I attach a photo of my latest effort; a repeat of the last example.

On glass with 2 coats of release wax, but without PVA release agent.


The tissue was CKS 6gsm

The treatment was:
   Zaponlac with thinners in 1:1 ratio sponged on
   3 coats of Pledge rolled on

Left to dry for a day. The edges of the tissue were prised off with a Stanley blade then easily peeled off the glass. The tissue was trimmed to a 120mm by 120mm square, to apply to a 100mm by 100mm square of foam. Again, care was taken to only apply the minimum of Meylan wallpaper paste to the foam. The tissue was laid down and my fingers were used to smooth it down from the middle to eliminate any air bubbles. The edges of the tissue were then folded over the foam and pasted.

I did not add any PVA glue to the Metylan.

The surface treatment only weighed 24gsm, 6gsm less than the previous experiment. Some of the saving must be the elimination of the release agent still being stuck to the surface. I am happy with the mass as it is half that of the previous treatment of sticking the tissue on then filling and sanding. It is similar mass to light aeromodelling tissues sealed over an open framework.

Unfortunately, whilst the surface has a lovley eggshell shine; it is not smooth. Small waves are present. I don't think that they are due to the wax. Perhaps, I need to experiment with other surfaces, as discussed by Paul.

The next photo shows the result of light sanding of a corner of the sample.




The sanding worked in flattening the waves after only a only few strokes of the 800 grit sanding sponge. But, I penetrated the tissue, which is unacceptable damage.

My ongoing test is in eliminating the initial Zapon coating. It is not going well, but I will report when finished.

Once I have achieved a good surface finish; I plan to switch to the lighter 4gsm CKS tissue.

My thought is that the tissue should act as a supporting mesh for the microfilm-like varnish. The lighter tissue should allow more flexibility. A key aim is for it to conform to gentle double curvatures. Experimentation with different varnishes is needed.
 

[Piecost]

I attach a photo of the Pledge multi-surface wax that I used so far.

In the middle is the Sand'N'Seal that I will try next.

On the right is "Polyvine" polyurethane, that was the cheapest/smallest quantity found on Amazon.