I had problems with grey many years back. But if let cure for about a week before putting fuel to it, solved the problem. I used it on the inside cabin area of a Decathalon. About a day later had a fuel tank leak when I filled up to test the engine. The grey wiped right off. Cleaned up and repainted with no problems on the inside. I used that grey later on the bottom of a P-40 with no problems around the exhaust area. Good point about the through holes. I never used it on firewalls, just thinned epoxy for fuel proofing.
Edwin