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Can anyone identifyfiberglass filler/primer by color?

bapboi

Member
I am in the middle of a minor engine cowling modification and the fiberglass work along the aft edge is complete. Sanding back the existing paint to blend in with epoxy filler I am uncovering something of a lavender color (best guess) under the paint that I cannot identify. I'm not sure if it's primer or if it is some type of body filler whether polyester or epoxy based. I was planning on using West systems epoxy resin mixed with their 407 fairing filler to smooth out the repair area but without knowing what this base material is I am reluctant to proceed. Anyone familiar enough with the commonly used fillers or primers to take a guess what this might be? It sands easily and is definitely not gray, pink, or brown...more like lavender...some slight purple tinge something.

Bruce
RV-7A (purchased)
 

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Smell It?

Just a guess, without being there....polyester and epoxy smell different when they're sanded. Polyester has a kind of sharp chemical-ly odor, epoxy, much less smell. Polyester usually (usually!) 'powders out' more when sanded. The color doesn't necessarily mean a whole lot, but I've never seen purple epoxy. Polyester automotive body filler (Bondo is the most common trade name but there are others) can have a variety of colors in the catalyst, I suppose lavender or purple among them, although blue is the color I'm most used to seeing. Another oddball test... try putting some super glue (cyanoacrylate) on it, let it cure and then try to peel it off. I've noticed cyano doesn't like to harden over polyester filler. I'm not sure if that's because of the polyester itself or the talc or what. Epoxy seems to be fine with it.
 
I've used US Composites microballoons on a couple of boat projects that dry to a similar brownish/purplish color. If thats what it is, they can be mixed with either epoxy or poly, so color isn't really an indicator of what you've got.

If you use epoxy it won't matter- epoxy will stick to poly just fine, but you typically cant go the other way.
 
If immediately under the top color coat, it is likely a primer or high build primer, and if so, it's probably a urethane. Doesn't matter, because if you intend to apply an epoxy/micro filler (407 is mostly phenolic microballoons, plus some silica to make it thixotropic), it's best to sand the underlying surface down to bare glass anyway.
 
Thanks for the input, I believe I have identified the substance in question. It appears to be West Systems 407 low density fairing filler, the product I was planning on using originally. I bought some of it and opened the can and it was indeed a dark reddish color, when mixed with epoxy it is a deep red and when hardened and sanded it lightens the color to the brownish red or lavender color in the picture. Best possible outcome for me as I now do not need to worry about compatibility issues between products. Thanks to all who took a guess!
Bruce
 
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