Lightweight Covering Materials FAQs

The purpose of this page is to try to answer those frequent questions that I receive. Hopefully it will save you time and ensure that I give consistent answers. If there are other questions that flow from these questions please ask.

Q1. Which way up do I cover with the heavyweight Polyester tissue?

A1. Cover so that the reinforcement threads are on the inside. Remember that you only need the heavy stuff on the largest models as it is very tough and quite heavy.

Q2 Which way up do I cover with the lightweight Polyester tissue?

A2. Cover with the shiny side outside. Be careful not to disturb the fibres when applying dope. Allow the dope to flow onto the polyester with the minimum of brushing, brushing will disturb the fibres.

Q3. Why does the dope stick to the Mylar when you dope tissue over it? On a convex surface it may not have to, but undercambered wings there may be a problem?

A3. Yes it will stick and there should be no problems. The gluing area is large and the load on the join minimal.

Q4. When the tissue goes all wrinkly from being wet with dope, do you let nature take its course, or some how ease the wrinkles out?

A4. Let it dry. If a wrinkle or two remain ease with more dope or iron through the tissue.

Q5. Do you dope "through" the tissue, or wet the Mylar and lay the tissue on top of it?

A5. I suggest that you attach with very thin dope through the tissue (25% dope 75% thinner). When dry apply more dope as required.

Q6. How do I keep the tip of my iron clean when covering models with heat shrink material?

A6. I suggest that you clean with the solvent that you use for the adhesive that used in the covering process. Simply wipe the hot iron with the solvent. Acetone or cellulose are equally effective in cleaning the tip of the iron.

Q7 Having water shrunk the tissue it has gone "tight as a drum" on all the flat sections such as the underside of the wing. However, on the more complex sections with compound curves I found it hard to get it tight. In some of these places the tissue still shows signs of slackness, even after water shrinking. Additionally there are some small "crows feet" wrinkles in corners where the top wing spar meets the ribs. Will the doping process generate sufficient additional shrinkage of the tissue to remove slight slackness and wrinkles? If not are there any steps I can take to rectify the problem prior to doping ?

A7. Basically Esaki tissue will shrink once and once only. The dope will not add much if any extra shrinkage. Actually it is better that dope does not keep adding shrinkage otherwise warps may well be introduced. There are ways and means of solving the problem. Firstly, if possible, remove the effected bits and re stick. Another way that needs care is a hot iron carefully applied to mould out the wrinkles. For the future covering activity I suggest that compound curves are best covered with lots of pieces of tissue. Covering with damp tissue can also be a way of avoiding such problems.

Q8. I have covered the model with Mylar over tissue and I have problems with warps and weight.

A8. You may have possibly used too heavier grade of Mylar. Keep to the thinnest Mylar on all lightweight structures. Progressive attempts at warp removal will result in the Mylar continuing to shrink exacerbating the problem. Similarly use less and much thinner dope when putting tissue over Mylar.

Q9. Anyone tried ink to colour the backside of clear Micafilm? I ordered some Polyspan dye, but discovered that Micafilm's instructions claim that dope won't stick to it [on the outside, where I'd hoped to spray dyed clear nitrate]. There was a thread about ink or dye being applied with a cotton ball, but don't recall the type of ink involved. Any other ideas?

A9. Quality ink from the art store is the stuff. Paint it on the furry side with something that won't give brush strokes, you may need two coats. Let it dry overnight before use. Do the inking on newspaper as despite the fuel proof claims the ink will bleed through. The finish looks like well doped tissue. Further details on "How -To-Do-It" number 13, see Publications

Q10. I know it has been mentioned here (on SAMTALK) a number of times, but, when covering with tissue, is the shiny side inside or out?

A10. Shiny side down. I read this years ago but I can't remember where. However it is not critical only don't mix the process or a difference in sheen will result. You won't spot the difference until under the glare of the sun on the flying site!

Q11. What weight Mylar tissue combination should I use on different structures?

A11. A broad rule of thumb is use the lightest Mylar with the lightest tissue and at the same time use less dope. For example if you previously covered in Esaki Liteflite and gave 4 coats of 50/50 dope/thinners. Then use 5 Micron Mylar and 2 coats 50/50 dope thinners.

Q12. Are the many differences that I should consider when using Polyester instead of tissue?

A12. Not really. The only real need to knows are:- Shiny side outside. Attach with either dope or contact then heat shrink, Polyester will not watershrink. Use minimum of dope as it is already waterproof and don't keep brushing otherwise the material will fluff up. You can cover top and bottom in a single piece by wrapping around the L.E. This makes joining and wrapping easier. Small dents iron out, you can weld slight tears with heat. If you break off a tip then heat and roll back the material, repair the structure then roll back the Polyester and reseal.

Q13. Is there a grain in silk? and what happens if I cover at 45 degrees to the weave?

A13 There is no grain as such in silk. It depends on how the stuff is woven. The warp fibres are length wise in the material and are stretched out for weaving. The weft fibres are then woven in at ninety degrees. It all depends on the tension applied and retained during the weaving. There are other patterns of weave, satin, crows foot etc. that are deliberately use an uneven weave. Thus if you use silk at other than at 90 degrees you could get a problem. The solution with double covering is to double cover in the opposite direction of the material if used at 45 degrees.

Q14. Can a wing be covered with just one long piece of Mylar for the whole bottom surface? Or, should I cover the bottom in four different stages with four separate pieces of Mylar? Prefer to do it that way to avoid a lot of seams, but is this possible?

A14. Cover the lower surface in 4 pieces and over lap over the dihedral ribs. This will not compromise the strength. One piece would induce a lot of wrinkles.

Q15. Several years ago I purchased a large quantity of nitrate dope. After some time the dope turned brown and hard due to interior can stain and solvents evaporation. Can The dope be salvaged?

A15. Dope ages regardless of how you might store it. It is a bit of a false economy to buy bulk. Buy fresh and bin when it goes off.

Q16. Lately have been using old stock (20 yrs old) of tissue on the Mylar .After pre-shrinking by weighting down the edges on top of a thick towel, letting it dry, turning it over and repeating, applying with glue stick etc., I notice that it just won't shrink up as tight as new tissue no matter what it's sprayed wth. The alligator effect/affect?

A16. Remember that paper ages. Some material will have a high acid content and will become brittle and lose it's properties after a number of years.

Q17. What are the pros and cons of Mylar with and without the tissue. Presumably on balance the tissue method is better otherwise no-one would bother with the extra effort.

A17. The Mylar on it's own is light, however it does not have any residual strength in terms of punctures and stiffness. The addition of tissue adds this aspect.

Q18. I'm having trouble getting smooth covering and wrinkles at dihedral breaks and wing tips.

A18. The answer breaks down into various sections:

The structure needs to be correct. There are lots of designed structures that are going to be at best a nightmare. The problem is that one cannot change the vintage stuff. Get the structure correct first otherwise it will be an uphill battle. Things that help are gussets on the surface at corners of the break. Thicker ribs are good, not angled ribs please as these distort both the section and the structure. A good way is to finish both wing halves with a thicker rib then sand off both to mate at the correct angle. You can then fix together and brace as you desire. The addition of cap strips at the break is another way of helping things. As to covering nothing will replace practice. The only way to attach tissue is with dope. Seal the structure first. Cut the panel to fit and try dry until it looks correct. Start attaching the covering one panel in from the break. When this is correct roll back the loose covering apply the dope and roll back and gently rub into place. Apply the other panel and overlap in the same way. Water shrink then dope. The biggest issue that I have had in this area is with tissue over Mylar. The Mylar will have shrunk in two directions much further than tissue likes to go when dry. My Ploy has been to apply as normal then iron over the tissue to smooth out the wrinkles. The iron on approach can be tried with tissue in areas where wrinkles can occur. Use thinned down contact adhesive than carefully iron and smooth in place. Water shrink and dope as normal. The main thing with all these ideas is practice and more practice. We are discussing craft skills and they are not won by simple tricks and devices, it is a learning curve. My first try with Mylar was several metres wasted and a wrinkled prune of a job. Now upteen tails later I can reel them off with nary a wrinkle in sight.

Q 19. How do you patch Mylar? Covered my Jerry Combs "Okie Dokie" with it, flew it outdoors (great flier, by the way, indoors or out), tore hole in wing.

A 19. Iron the damaged area. This should pull it back in shape. Iron on a piece of pre glued Mylar. The ironing of the damaged area will sort out any dents and will pull things tight around the hole. Often the iron will pull back what you thought was a recovering job. Take a section of unused material and paint with adhesive and leave to dry. Once dry cut out a piece the correct size to cover the hole. Tack in place around the edge then iron down and shrink. I keep a chunk ready and done, when doing a new model and if there is adhesive left I top up the patching roll.

Q 20. How hot should the iron be when shrinking Mylar?

A 20. The iron needs to be at 200c to shrink the Mylar. The iron can operate at its maximum without fear of melting the Mylar.

For details of availability of the materials see Lightweight Covering Material

See also Publications - "How-To-Do-It" No's.1, 4, 5, 10, 13, 14 for help in using these lightweight covering materials