Repair foam-core wings: A quick and easy fix
Model aircraft are made of a wide variety of materials: they may have built-up balsa construction or be made entirely of foam. Many have a fiberglass fuselage and balsa-sheeted foam-core wings. These are quite durable, and this is fortunate because fiberglass fuselages and foam-core wings are somewhat more difficult to repair than balsa.
My Little Toni 1/4-midget pylon racer suffered from a fuel seepage that dissolved part of the wing’s foam-core. This weakened the wing greatly, and I was forced to retire the model. After it had sat in the rafters for years, I decided to pull it down and repair it. The following steps describe how I mended the middle of the foam-core.
* Hobby knife or saw
* Needle-nose pliers or tweezers
* 5- and 30-minute epoxy
* Insulation foam in a can (the minimal-expansion kind)
* Palette knife or Popsicle stick
* Thinning alcohol
* Fiberglass cloth
* Masking tape
* Wet-and-dry sandpaper, 320- and 600-grit
* Tack cloth
* Rubbing alcohol
* Electrician’s tape
* Brown wrapping paper
* Primer and matching paint
2 The damage was caused by fuel seeping in through a crack and dissolving the foam-core. Very little of the wing’s outer surface was damaged, but the damage in a central high-stress area must be repaired. Begin by cutting away the balsa skin to expose the damaged foam, which you then remove with needle-nose pliers or tweezers.
3 This wing’s underside also has small cracks and splits; fill such cracks with 5-minute epoxy and let it cure fully. Slowly spray a little canned foam into the damaged area. Don’t use a lot of foam; if you spray too much into a model structure, its expansion will cause damage; a little goes a long way! The foam will fill the cavity, and any excess will overflow and cure on the outside of the wing. Let the foam set for a couple of minutes, and then remove any excess with a palette knife or a Popsicle stick.
4 The foam continues to expand until it has completely dried. Remove the excess dried foam with a hobby saw or knife and then sand it flush with the wing’s surface. Apply a coat of 30-minute epoxy thinned with alcohol over the repaired area, and cover it with a layer of fiberglass cloth. Here, I covered the entire center section. Brush on a second coat of thinned epoxy to fill the fiberglass cloth’s weave, and let it cure completely. Use masking tape to prevent the epoxy from getting on surfaces where it isn’t required.
5 After the epoxy has cured, sand the repair with 320- and then 600-grit wet-and-dry sandpaper. Feather the edges of the fiberglass cloth so there is a smooth transition between it and the rest of the wing. If you sanded too much in some areas, apply another thin coat of epoxy to fill them, and sand them smooth when the epoxy has cured. When you have sanded the repair smooth and the fiberglass cloth’s weave doesn’t show, you’re ready to mask and repaint the repaired area.
6 Remove the protective tape, and use a tack cloth to wipe away any sanding dust left on the wing. Clean the wing with rubbing alcohol, and use electrician’s tape to mask the repaired area. Electrician’s tape leaves a cleaner edge than masking tape, and very little paint will seep under it. Use brown wrapping paper to protect the rest of the wing from overspray.
7 First apply two coats of primer and let it dry. Wet-sand between the coats to remove most of the primer before you spray on the next coat. I sprayed on a total of four coats, and when the final coat had been allowed to dry overnight, I wet-sanded it with 600-grit wet-and-dry sandpaper. To prevent the primer from sticking to the tape’s edges, lightly sand its edges before you remove it. Let the wing dry completely after you wet-sand it, or the moisture will cause the paint to blush. Spray on two light finish coats, letting the first layer dry before you apply the second one. Apply one last, slightly heavier coat so that the surface of the repaired area will flow outward and produce a “wet” appearance.
8 When the paint has dried completely, remove the masking tape and paper.
To ensure a sharp paint line, remove the electrician’s tape by pulling it back slowly over itself. This action cuts the paint smoothly and avoids making jagged edges. You can hardly see where the repair ends.
That’s it; sit back and admire your repair. My Little Toni is ready to race again, and you’d never know it had been damaged. With some simple tools, supplies and a little time, you can easily repair a damaged foam-core wing; and remember, it’s always better to repair a wing than to build a new one!
LustreKote; distributed by Great manes Model Distributors (800) 637-7660; greatplanes.com.
Kwik Bond Epoxy; distributed by Global Hobby Distributors (714) 963-0329; globalhobby.com.
Copyright Air Age Publishing Jun 2003
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