PETG filler and alternatives to sanding  

PETG filler and alternatives to sanding

Hi, I have a project where I'm printing lightweight PETG parts (1 wall, 6% infill, heavily optimized), with the intent to encase parts in fiberglass-epoxy composite, similar to how surfing boards are made, but with 3D prints instead of foam. PETG was chosen because of it's low price and decent temperature resistance. Printed parts sometimes come out with some small defects and low spots that I need to fill in with some filler, and I can't find anything specific that works well with PETG.

Filler must obviously adhere well to PETG (even if it is not keyed with sandpaper), sand easily (so it must be soft), and epoxy should stick to it. Any recommendations? I suspect car body filler (polyester/vinyl) would probably work, but it's a mess to work with. Maybe there are some compatible 1 component fillers? I tried CA glue, and it works, but it's much harder than PETG and difficult to sand flush afterwards. Trying to mix CA with powder only makes it harden faster than I can spread it.

Second question, is there any alternative to sanding to make epoxy stick to PETG? I have ~20kg / 50 m^2 of parts, and sanding them would be a huge chore, even with power tools, especially because of how delicate they are with that 1 wall. Sandblasting is not an option unfortunately. I read that licking PETG with a flame should reduce it's surface tension, but I tried it, and it didn't seem to work, and it's very easy to distort the parts with this method. Are there any alternatives?