As to the styrene foams, if you're just building a quick plug, cover
the foam with a thin layer of FixAll plaster, then paint, wax
and lay up with polyester or whatever.
Later,
Doc
Captain Nemo wrote:
Well, I'm working with epoxy and glass right now, and it's still too wet to go to the next step, so here's a few things I think I know about this sort of thing, in case anybody is interested. Let's see...where to start? Some guys use either styrofoam or (even better) extruded polystyrene foam (pink or blue insulation from the hardware store) as the basis for a "plug" from which to make a fiberglass fairing shell from. This is a good way to go, but there's a few things to know about it. 1. Bondo will "eat" (dissolve) these kinds of foams. Bondo and some fiber glass resins contain chemicals like styrene monomer which dissolves styrofoam and polystyrene foam. If you make a shaping error and need to fill a ding in this kind of foam, don't use Bondo. Instead, scuff it with coarse sandpaper and pour in polyurethane foam. It will bond to the substrate without dissolving it. A little goes a long way: "surfboard foam" starts as a liquid and expands. Once you fill the ding, sand it as you would any other foam. 2. Fiberglass resin will also eat styrofoam and polystyrene foam (but not polyurethane foam). To protect styrene-based foam from being dissolved, coat it with epoxy. Epoxy doesn't heat up like resin does, so it won't melt the foam. It doesn't dissolve foam, either, because it doesn't contain styrene monomer. But it is expensive, whether you buy the amber stuff that comes in two tubes, or get it in gallon or larger cans, the price is high in comparison to fiberglass resin. Worth it if the project requires it, though. 3. Generally, curing time is a factor in determining strength. Slow curing epoxy is a lot stronger than the fast curing stuff. If you want to put a good strong protective seal coat on styrofoam or polystyrene foam, use the slow-curing epoxy. 4. Once you have the foam sealed, you can cover it in fiberglass. Scuff the epoxy with coarse sandpaper, and apply the glass. Then, sand smooth, apply your finish, and you've pretty much got a plug from which a female mold can be made. 5. If you just want a one-off fairing from this process (and don't want to make a mold) coat the ourside of the foam plug with several layers of fiberglass cloth for strength. Sand the exterior and get it as smooth as you want it to be. Then, dissolve out the foam core inside with a foam-eating solvent. What works for me is 100 octane low-lead av gasoline (airplane fuel, available at the local airport). It will melt the foam away; doesn't leave a bad smell when dry; cleans up easily with soap and water; and is relatively inexpensive as solvents go. 6. Once the foam is gone, if you want to strengthen the fairing, apply some fiberglass matting to the inside surfaces to thicken it up a bit. OK, I gotta get back to work. Any body want to add to this thread on working with 'glass? Pat