Covering Blue Foam

[RolandD6]

I have not left hospital yet but I can make to comments:

1. Yes a shiny surface is important but more for appearance of the tissue surface.

2. Probably the most important feature of the backing surface is LOW SURFACE ENERGY.


My transport has arrived so more later.


Paul

[RolandD6]

To continue:

There is clearly a relationship between the supporting surface and the fluids being applied to seal the tissue. You are reporting success with glass which is opposite to my experience. I had the most success with HDPE which is an acronym for High Density Polyethylene. Kitchen shrink wrap is often LPDE which is an acronym for Low Density Polyethylene. Some kitchen shrink wraps are made of PVC and perhaps other plastics are used around the world.

HDPD is a plastic typically used to make milk bottles and it is translucent, not clear like window glass.

In Australia it is typically sold in 2400 x 1200 mm sheets but I was able to find a local person on eBay who was cutting the large sheets into smaller pieces.

I will attach a slightly modified copy of the report I posted on the old HPA web site which may be helpful to you but keep in mind it was a work in progress at that time. I may go back to it.

Tengujo is a brand of Kozo tissue

To be honest, there is not a lot of advantage I working with 5gsm Kozo and perhaps marginal advantage working with 6 gsm Kozo. The weight problem you have to overcome is the weight of the fillers you mention.

Eg: Mica, a common filler in "metallic" ink, has a specific gravity of around 2.7 which is close enough to being the same as real aluminium powder, ignoring inter-granular spaces.

Glass is similar as is talcum powder and calcium carbonate. A specific gravity of of 2.7 means 2.7 times the weight of pure water, so 2.7 gm/cc.

The follow was extracted from a post I placed on the old HPA website.

The model under discussion is my avatar image.

This model began life as a Porterfield CP-50 about 14 years ago and is now an
LP-65. The most significant difference in the types is the engine but all types
had variations such as roof glazing or not for example. CP-50's were fitted with
a Continental 50 and the LP-65's were fitted with a Lycoming 65.

The full size airplane has a wingspan of 34' 9" (according to Juptner) and the
model is 1/20 scale giving it a span of just under 21". Juptner says the wing
chord is 60" but the 3-view drawing (actually of a CP-65) I used scaled to 3.25"
Going by Juptner saying the wing area of the full size is 168.8 sq.ft., the model's wing area is somewhere around 61+ square inches. In its previous life the total weight of the model was about 25gm equating to a wing loading of about 0.41 gm/sq.in. I cannot remember if that weight included rubber.

The refurbished model now weigh 26.8 gm including rubber an a 6.5" plastic propellor. Without rubber or propellor; the fuselage, undercarriage, wings and empennage weigh 17.5 gsm and the original nose block and propeller assembly weigh 4.4 gm. The original laminated propeller broke in the crash.
Longitudinal balance seems to be much the same as before so I am not
expecting too much trauma during the first test flight. This time I have fitted an adjustable rudder but the tailplane is unchanged, adjusted with packing.
Please note:
The following discussion may not appeal to people who can used volatile
materials like cellulose dope and air brushing solvent enamels. I cannot so I
have been working on methods using water based products like acrylic inks, acrylic floor finishes and perhaps water based polyurethane lacquer. I can cope
with alcohol solvents like industrial quality methylated spirit.

The discussion also is about tissue paper lighter and more porous than
traditional Esaki.

The original covering was a Japanese tissue known as Sagigawa which has an
average weight of 9 gsm.

The refurbished covering is 9 gsm 'unbuffered' tissue of unknown brand
purchased from a fancy art and stationary store. Rolls of similar
tissue can be found on the internet if you search for 9 gsm unbuffered tissue.
The tailplane was covered with a different tissue as an experiment but I am not
satisfied with the result so I will not describe the process.

Originally the fuselage was covered dry with tissue pre-coloured by spray
painting with pigmented ink. The tissue on the wings and empennage was pre-
painted with black then silver and applied to the frame dry. The result was not
exactly drum tight and the black undercoat was a mistake. I was following the
recommendations of plastic model builders. I later found a dense black under-
colour was not needed to get a pleasant enough and much lighter result.
Perhaps not to a competitive standard but good enough for me.

The refurbished covering was also pre-prepared away from the model. The
empennage was covered with dry tissue and the wings and fuselage with wet
tissue (water spray). The wet tissue tightened up nicely as it dried. The
registration markings were applied to the tissue as part of the pre-covering
preparation.
All of the tissue was prepared on top of smooth 1.5mm thick HDPE plastic sheet
purchased from a local supplier via eBay. For this process I have tried various
types of polypropylene (PP) sheet, silicon rubber sheet, EVA plastic sheet, LDPE
film and HDPE sheet. The PP and EVA was unreliable, the silicon rubber did not
work, the LDPE worked well but was tricky to handle and overall the HDPE gave
me the best result.

The treatment process is as follows:
1. Ensure the work surface is free of contaminates and serious scratches;

2. Lay the tissue paper onto the work surface and brush on ordinary potable tap
water. There generally is enough gunk and surface tension in the water to cause
the tissue to stick to the work surface;
3. BEFORE THE WATER IS FULLY DRY, brush on the first treatment coat of 10%
acrylic ink and 90% water. This will stick the tissue to the work surface. The ink can be coloured or clear depending on the required end result;

4. Apply 3 or 4 coats, (perhaps more if the tissue is to be applied dry with no
further sealing), in the same manner ensuring you alternate brush stroke
directions. You could airbrush the same mixture to get a better result but more
coats may be needed;

5. Next I normally glue a strip of 1/4" square hard balsa strip to two only
opposite edges (usually the longer edges) as an aid to handling, especially if I
choose to wet cover the frame. Thin cheap PVA generally works fine but this
time I used low strength school gum, not PVA;

6. Run a thin artist painting knife under the balsa strips to release them from
the work surface;

7. Use the balsa strips to gently pull the tissue back onto itself to release it from the work surface. Some of the acrylic ink may remain on the work surface.
Some inks do, some not so much. I have yet to find a reliable way of reducing
the stickiness of acrylic ink without adding much weight. Some tests have been
promising. (the surface can be cleaned with original Windex, containing ammonia).

For the wing covering I skipped step 4 and proceeded to apply the registration
markings using Faber Castell Pitt artist pens with the HDPE worksheet on top of
a pattern printed on tracing paper over a light box.
For the fuselage covering I completed step 4 before applying the registration
markings with Pitt pens.

I glued the flat empennage frames to the dry pre-prepared tissue before
removal from the work surface.

For glue I used a Japanese tissue paste purchased from an art materials supplier. I also use the same paste when applying the wet tissue to the fuselage and wing structures.

At the completion of covering, the wings and fuselage were air brush coated
with an acrylic floor polish similar to Johnson's Future. The fuselage got three
coats and the wings four. Each coat was just enough to make the tissue go
baggy.

The empennage was not coated with floor polish to keep the weight down and to
avoid the two tissue sides sticking to each other.

Having read comments about the shrinking capability of Eze Dope I conducted a
comparison experiment between a 15% solution of Eze Dope and full strength
floor polish. The results were very similar so I decided to go with the floor polish because it is much cheaper than Eze Dope.

The next block of text describes another experiment I did with 5 gsm Kozo tissue.

Tissue: Nominal 5gsm Tengujo (Kozo) tissue.

Materials:
   Water;
   Windex containing ammonia;
   Matisse MM8 Spreader Medium, (lowers the surface tension of acrylic inks and
   paint);
   Peerless Jal Gemini Floor Finish (gloss) or equivalent, (This an industrial
   acrylic polish for vinyl floors, similar to Johnson & Johnson Future);
   Jacquard Dye-Na-Flow colour;
   Faber Castell Artist Pen (white);
   HDPE plastic sheet.

Procedure:
1. Lay the tissue on the HDPE. An option is to wet it with water to pre-expand
   it, but let the tissue nearly dry before the next step;

2. Brush on one coat of this mixture:
   50%(20% MM8, 80% Gemini)
   50% Windex, let completely dry;

3. Brush on one coat of this mixture:
   33.3%(20% MM8, 80% Gemini)
   66.7% water, let completely dry;

4. White stripe applied with Pitt pen. It appears
   the tissue is too fine for this because the
   alcohol in the ink etched through to the HDPE;

5. Apply one layer of coloured Dye-Na-Flow.

Those mixtures were to only ones I tried in this part of the project, there may be better. The result was a very nice coloured surface that was about 9 gsm.

That is about all that I can add to this discussion unless something else prompts my memory.

Paul

[lincoln]

Paul:
You mentioned methylated spirits. Have you experimented with shellac?

[RolandD6]

No

That was going to be next on the list. I have some blond shellac flakes and industrial methylated spirits. Normal hardware shop metho can contain about 5% water as well as 5% methanol.

I should also add many plastics like acrylic ink and paint have a specific gravity around 1.1 so about 1.1gm/cc. That is ignoring any pigment. Titanium oxide (white) has a specific gravity of about 4.25

[Piecost]

Paul,

Thank you for posting your experiences. There is much to digest.

Tim