How to Fiberglass Cowlings
By Edwin Smith
You should make a few practice cowls first before the real thing.
1) Tack glue a styro-foam block to the firewall. I use Elmer's white glue spread out like 4 or 5 dots. Remember, you have to break it off eventually. Sand to shape with what ever means seems practical. A rasp takes a lot off to rough it in quicker. Then a sanding bar with 80 grit does a good job finish sanding. I normally sand even with the fuse side so that the cowl will fit over the fuse like a plug.
2) Paint a coat of white Elmer's glue thinned with water onto the styro-foam. This will seal it so that you don't get a bunch of little spikes on the inside of your cowl. When dry, fill in the voids with light weight spackling. I apply it with water on my finger so it will fill all the voids. Sand again. Paint another coat of thinned white Elmer's glue on and let dry. If you make the cowl to plug onto the fuse you need to protect the fuse. I use a clear mylar packing tape. Make it wide enough to keep resin off the fuselage. I overlap the fuse with glass by at least 3/4". This leave me enough to trim the edge.
3) I use 4oz to 6oz fiberglass cloth to make cowls. You can get it from West Marine, any boat repair shop, Wal-Mart or AutoZone. Its a heavy weave that goes around corners pretty good. You can spray a light dusting of 3M77 spray adhesive on the plug if you want to, this will hold the cloth in place while you apply the epoxy or finishing resin. Lay a piece of cloth on. Apply layers in pieces changing the orientation of the weave. You can use 30 min epoxy THINNED A LITTLE with denatured alcohol from the hardware store, you don't want it too runny. Or you can use Finishing Resin from the hobby shop. Finishing Resin cures a little stiffer than epoxy and is good when making wheel well doors. Epoxy does fine for cowls, it stays a little more flexible which is handy for grinding the plane into the dirt. Others use West System Epoxy, but the mix is not 50/50. It comes with a squirt system that meters out the correct amount. Mix small amounts. Cleaned up pudding cups about 1/3 filled is about as much as I want to mix at a time. I use disposable hobby brushes to paint on the resin. Disposable tin handle brushes can be bought at Harbor Freight or ordered in bulk from Enco at pretty cheap prices. I let a coat cure till its a little tacky then apply another layer of cloth and resin. Make sure the weave is thoroughly saturated but not dripping. I do this till I have 3 layers on. Let cure till pretty hard. Usually over night is enough.
4) Before sanding, make sure you have a respirator and goggles. Fine glass fibers are pretty hazardous to your health. You can sand with the plug still on the fuse or just break the tack glued plug off. Trim the overlap on the fuselage before breaking the plug off. If its too long it stress the glass work or can damage the fuselage. I usually do about 3/4", it should make a clean break. (Unless you used too much glue) I use a Dremel with a cutting wheel to trim it. Just do a rough cut to get it off then do a finish cut when you're done with cleaning. Be careful, it takes a steady hand. Sometimes you might want to use Ever Coat Fiberglass Bondo to help get a finer finish. It can be purchased at an auto parts store. It stays pretty flexible after its cured. Mix up small amounts. It hardens pretty quick and can be sanded in a couple of hours. I normally end up sanding off about 1 layer of cloth by the time I'm done.
5) To remove the foam, just pour a little gasoline onto the styro-foam. It will dissolve it. Do this until its all dissolved and just scrape out the messy goo left inside then rinse with water. I carefully use a narrow paint scraper. To get into the tight spots I use a Dremel with a small (~1" dia) wire wheel and a crown type wire brush. Its easier to let the cowl dry before you use the Dremel. You will also be scraping out that last coat of Elmer's glue. The Elmer's glue is what helps to give a smooth finish on the inside.
6) I have used Luster coat and Rust-oleum to paint it, but I guess you could use just about anything that's fuel proof.
General suggestions:
RETURN TO BUILDING TIPS1) One improvement I will do next time is to imbed a piece of 1/16" ply or carbon fiber in the areas that the screw holes will be. Bad vibration will damage it in the area of the screw holes. You can also use whatever weight glass is available.
2) In areas that will have cutouts you can build it up thicker. That can be done during the lay up or you can wait till the inside is cleaned out and build up more layers on the inside.
3) To line up the crankshaft I will make a thrust line on the fuselage sides top and bottom. To do this apply some scotch tape that can be marked on. You basically setup a cross hair center on the firewall. After attaching the styro-foam block I will sand the sides top and bottom straight. Then use the thrust lines on the fuselage to extend onto the foam block. I haven't done it yet but I think a cheap laser of some sort would simplify that. Mark your hole and start sanding from there. Sometimes I put a ply disk where the crankshaft goes for extra support. Its not really necessary though.