Rocketman320

In this thread, I want to concentrate the discussion on the lightweight silk covering techniques with either wbpu, glue, or resin. I have three planes to cover - 2 foam and one balsa. I am looking for the best method to achieve a lightweight, strong, paintable surface that will yield an excellent gloss finish when painted. I've decided against glass and resin (weight) and liquid sheeting (fumes) A few questions:

1. What are the best methods so seal the balsa and foam?
2. Wbpu or glue - pros cons?
3. Silk and low viscosity resin -has anyone tried this?

Thanks in advance for you time and information.

wyowindworks

Are these open (built up) structures or closed surfaces?

You will have to sand the surface to prep for paint. When you sand the silk it will fray. You can get glass fabric (0.56 oz/yard) that is just as light or lighter than silk. Also, the resin will chemically bond to the glass via the finish. The silk will only get encapsulated. In my opinion, light glass and epoxy will seal the grain and cells (foam) best. If you spritz the balsa with Rave hairspray (aerosol can) that balsa will soak up less resin. This trick doens't work with WBPU because it will disolve the hairspray (water based).

Rocketman320

Thanks for the information. How would wbpu and lightweight spackle work as a wood filler for foam and balsa. I am lookinf for a super lightweight filler that will seal up all foam and balsa so there is minimal resin absorption. I really need to keep things light but achieve a strong hard surface.

wyowindworks

The WBPU is going to soak in much more than epoxy resin. If you don't add some glass then the grain/cells will swell with changes in temperature. All of sudden your "glossy" finish won't be flat and smooth anymore. The advantage to glass is that the fibers will span the grain making the surface stable. If you use .56 ounce glass and epoxy you will adding around .001 ounces per sq. inch (.028 grams). If you use 1 ounce glass + epoxy you will add around .0016 ounces (.0454 grams) per square inch. This includes sealing with Rave Aerosol hair spray to seal the surface before doing the lamination. The paint and primmer will end up adding more weight than the glass and epoxy (if done correctly).

Rocketman320

Will the Rave have an adverse effect on the foam structures? What do you recommend for filling in panel lines before glassing? Also, I have heard from other forums that glass and resin, when applied properly, will be one of the lightest finishes going. This was confirmed by direct weight comparisons. What technique do you use to apply glass to keep things as light as possible? Thanks!

Johnny U.

wyowindworks

First off, let me make a correction. You want to useAqua Net hairspray not Rave. Sorry about that. The Aqua Net will not attack the foam. Although most foam does not absorb very much resin so it is only really needed with balsa.

The application style really depends on the shape and the available equipment.

The best/lightest application of glass, epoxy & paint is to use mylars and a vacuum bag. The mylars get waxed and painted. Then glass and epoxy get applied to the mylar. The myler is then transferred to the surface and then placed in vacuum bag. Once cured the mylar peels off the layup leaving the paint bonded to the resin/glass. The paint finish mimics the mylar finish so it ends up nice, glossy, and flat (see photo of a part I made today). If the shapes have a lot of complex contours this technique does not work very well because the mylar is unable to conform properly. It works great for wings though. Your structure also needs to be able to handle the pressure generated by the vacuum bag.

A technique that works better on complex shapes is to mist the surface with 3M 77 Spray adhesive, carefully lay on the glass (use a paper carry for really light glass to prevent wrinkling), brush on the resin, absorb the excess with paper towels, and let cure. Once cured the surface gets sanded, lightly filled with resin + microspheres, sanded and painted.

If the surface isn't that complex you can just drape the glass over the surface, brush on resin, absorb the excess with paper towels and proceed like above.

The heaviest part of the whole thing will be your primer and paint.

Any panel lines can be filled with light weight spackle....like for filling nails holes in walls before painting.

What are you trying to cover? Pictures? That would really help.

Rocketman320

Thanks for the excellent advice. Here are links to the planes I am building: a Multiplex Twister and a Multiplex Fun Cub.

Twister
http://www.horizonhobby.com/Products...odID=MPU214222

Fun Cub
http://www.horizonhobby.com/Products...odID=MPU214243

The other plane that I need to cover are two old Mustang 450 kits that consist of a balsa sheeted foam core wing and a balsa fuselage. Here is an old picture attached below. It is a pylon racer that I am converting to electric for use as a sport plane - always loved this plane as a kid.

The twister has a lot of compound curves so I'll have to use the spray adhesive technique them mop up the extra epoxy with paper towels or toilet paper. I really have to keep thins light. I've looked at paper and glue, wbpu and silk/glass, etc. In all cases, guys that done both and compared weights have all said that properly done, glass and epoxy yields the hard and best finish. Even with a lot of wbpu and glass planes, the still look like foam in many cases. There are a few posters that have said epoxy/glass does not work well over foam - any ideas why?

wyowindworks

Epoxy and glass works well over foam but you shouldn't expect it to very tough at the weights we are talking about. When it comes to dent resistance the composite skin is only as strong as the compression strength of the foam.....unless you add a significant amount of glass. .56 ounce and 1 ounce glass are going to provide very little resistance to dings and dents. Done correctly, it will provide a nice foundtion for painting though.

Mrs R

BAck in the 60's and 80's we used a lot of silk as a covering but used dope 2 or 3 coats first then filled any dents efc with a mix of Dope and Talc (non scented) lightly sanded another coat of Dope and then layed the silk on wet (damp with water) stuck it down around the edges with Dope let dry then brushed a thinned coat of Dope all over to glue the silk down then a couple more coats lightly sanded between coats and then prime etc.Worked wellSome thing to think about.
BESTREGARDS
Rachael

kenh3497

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Here is MY experience with glass and epoxy. First a bit of background. Back in the day when 1/4 midget racing was hot I built a Caudren. All wood. I used 3/4 oz glass cloth and epoxy to finish. Min. weight of a ready to fly airplane without fuel was 2 1/2 pounds.

OK, As usual I did all the steps to get the surface as good as possible before glassing. I used the 3/4 oz cloth and stuck it with the epoxy thinned a bit with denatured alcohol. I had it thin enough so it would spread easily with a playing card. When spreading the epoxy, you could look at the surface and tell if you had “dry” spots because the “looked dry”. If you had too much epoxy it looked wet and shiny. I don’t know how else to explain it.

After the epoxy cured I very lightly sanded the first layer with worn out 600 just to knock off the “fuzz”. I then applied another coat of epoxy thinned slightly more than the second. Again a very light sanding to knock off any fuzz, there was very little this time. Now I primed and painted as usual. I used lacquer at the time and it worked well.

When the plane was complete I had to add 2 ½ oz of weight to make the min. weight. I still have the Caudren and though it has a bit hanger rash it still is looking pretty good.

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This technique worked well for me, but hat has been a very long time ago. I'm sure the epoxies have changed since then... I will delve back into this in the next year as I plan on building a 20% Zlin 526.

Ken