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Greetings!
I was hoping that you folks might be able to tell me if it is possible to "glass" over the blue foam I have seen other scratch-builders use in forming masters. I do not know which (if not all) of the fiberglassing products will attack the foam. Is there an alternative product that will work? Thanks for the input in advance.
 

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I really like working with blue foam.

As Fernieo says, Epoxy does work well. I like some of the epoxy modeling putties such as MagicSculp/MagicSmooth, Apoxie Sculpt, etc.

These are easily shaped.

I also like using the foam putty favored by RC plane hobbyists for build-up and repair work, then just using liquid epoxy over that and the main foam for the final layer.

Some time ago, I did a study on what were the best foam covering putties, and here's the summary I've posted before.

Over the last few months, I've been on a quest for the ultimate foam coating. I'll share what I've found so far.

I performed the following experiment on the leading contenders.
I took a piece of regular blue foam (DOW Syrofoam 1/2" insulation foam) about 4" wide, and applied the surface coating to the end. I then evaluated each surface coating material on the following criteria.
- price
- availability
- mixing
- wet odor
- application
- wet smoothing
- cast-in impression (I pushed a round head of a pin into the gelled coating to cast in dimples)
- curing
- filing
- wet sanding
- cured odor
- adhesion (I cut through the surface material at one end then ripped the material off to see if it just flaked off or took foam with it; this also tested brittleness)
- scribing
- dent (I pushed the flat end of a Bic pen against the surface to see if it dents and/or fractures the surface)

Here are the materials I evaluated:
- Spackle
- Latex wood filler
- 5-minute epoxy
- marine epoxy resin
- Aves Apoxie Sculpt epoxy putty
- MagicSmooth epoxy paste
- Bondo Home Solutions wood repair epoxy putty

Here's how the materials fared.

Spackle: least expensive; readily available locally; pre-mixed; slight, unoffensive odor; easy application; smooths with a wet finger; accepts a dimple; slow to dry; no need to file; sands very easily (perhaps too easily); no noticeable cured odor; poor adhesion (flakes off) (the test also caused fractures); won't hold a scribed line; dents easily (the Bic impression is actually readable); overall: poor.

Latex wood filler: mid-priced; readily available locally; pre-mixed; slight, unoffensive odor; easy application; smooths with a wet finger; accepts a dimple; slow to dry (thicker applications never dry properly); no need to file; sands very easily; no noticeable cured odor; worst adhesion (the test also caused fractures); not good for scribing; dents/fractures easily; overall: poor (slightly better than spackle but not enough so to justify the higher cost).

5-minute epoxy: about $1 per fl oz; readily available locally; measures and mixes easily; unpleasant odor; easy application; self-leveling (but with miniscus) but still somewhat thixotropic; wil not accept dimple (sticks to the instrument, too thin for this purpose anyway); dries fast but remains tacky a while (fine overnight); files ok but does gum up the file some; sands smooth easily (though it's important to let it cure overnight first (otherwise too gummy)), hard to sand down appreciable thickness; odor is noticeable during sanding; good adhesion (the test also caused a fracture, though); good for light scribing; dents very slightly and only after much force; overall: surprisingly good, but too thick for simple sealing coat, too thin for a putty (for sculpting).

marine epoxy: about $1 per fl oz, shipped; I had to mail order (I got mine from System3) (Home Despot used to carry it and it was cheaper; alas, they stopped, and I couldn't find it locally); measures and mixes easily (mine was 2:1 ratio); no noticeable odor; easy application; self-leveling and goes on thin; wil not accept dimple (sticks to the instrument, too thin for this purpose anyway); slower to dry but fine overnight; no need to file; sands smooth easily; no noticeable cured odor; good adhesion; good for light scribing; dents with moderate force; overall: perfect for a thin sealing coat (but it can be dented).

Apoxie Sculpt: about $1 per fl oz, shipped; mail order; measures and mixes fairly easily; unoffensive odor; works like clay, not particularly easy to apply a surface coating (doesn't like to stick, also a bit thick for this purpose) (Joe Brown reports that it works well if rolled out very, very thin and draped over the surface); smooths with a wet finger; accepts dimple and holds it very well when cured; overnight cure; files fairly well (it's pretty hard stuff, though); wet sands nicely (hard to take it down far, though); no noticeable cured odor; good adhesion; good for light and heavy scribing; will not dent; overall: perfect for a thick, sculptable coat.

Magic Smooth: about $1 per fl oz, shipped; mail order; harder to measure out equal amounts (though still readily accomplished), mixes fairly easily; unoffensive odor; thinner but stickier than apoxie sculpt, fairly easy to apply a surface coating (sticks well); smooths with a wet finger; doesn't accept dimple (sticks to the instrument); overnight cure; files fairly well (gums up file a bit, though); wet sands nicely (hard to take it down far, though); unoffensive, slight cured odor; good adhesion; good for light and heavy scribing; will not dent; overall: good, but I prefer Apoxie sculpt for the thick and marine epoxy for the thin; I'll use what I have but probably won't buy more.

Bondo wood repair epoxy: about $1 per fl oz; available locally (Lowes); easy to measure and mix (less sticky than MagicSmooth); unoffensive odor (a bit like enamel paint); just a little thicker than magicsmooth and less sticky, spreads like butter; can't smooth with a wet finger (it sticks); doesn't accept dimple (sticks to the instrument); overnight cure; files fairly well (it's hard stuff, though); wet sands well, but it's very hard stuff, which makes it hard to take it down much; slight cured odor (like enamel paint); good adhesion (the test also caused a fracture, though); good for light and heavy scribing; will not dent; overall: good, but I prefer Apoxie sculpt; main disadvantage (compared to Apoxie and MagicSmooth) is that it doesn't smooth with a wet finger; I'll use what I have but won't buy more.

I'm now convinced that the perfect smoother is actually a pair of materials, one thin (paint it on) for just sealing the surface, another thick but easy to sand (ideally as easy as the foam itself) to use as a filler and build-up material. The best I've found for the former is marine epoxy; the best for the latter is Apoxie Sculpt, though it isn't nearly as easy to sand as I'd want. I've considered using spackle for the filler material then subsequently coating it with the epoxy liquid, but I haven't tried that yet (and I don't like its poor adhesion). I've also thought of using polyurethane foam as a filler material, though I think the bother, mess, and fumes will detract.
One hobby shop near me has a tub of ready-to-use lightweight filler for use with foam or balsa RC plane wings. It claims to sand and finish just like balsa, and this is basically true. It was quite pricey (especially compared to the blue foam, but also compared to Apoxie sculpt).

I also plan to experiment with different fillers and mixtures (silica thickener, phenolic microballoons, plastic minifibers, wood fibers, etc.) later in my "quest." [Still haven't gotten around to this]

p.s. I've also tried white glue as a surface sealer. At first, it seemed to work ok, but it didn't seal the surface particulary well. When I applied glazing putty over the sealed surface as a test, it destroyed the foam right through the white glue barrier.

p.p.s. It probably goes without saying, but glazing putty (e.g., the Bondo tube) and polyester resin don't work as surface coatings; these things eat the foam for lunch. (You may have seen Joe Brown's pictures demonstrating this.)
 

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I believe REL uses automotive Bondo on all those impressive starships he's made. :thumbsup:
 
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