RE: thinning epoxy
Thinning epoxy I don't. westsystems.com has detailed methods to thin without adding anything. Acetone is what I would use if anything for non structual repairs but never for building a plane. They use it for rotted wood repairs in boats where water has turned the wood under glass to black mush.
Thinning polyester resin is the norm and I had to fight the urge when using zpoxy the first time and now West systems 105/205. Working it in with a squeege or brush and let it gell before trimming the excess.