Back to the epoxy thinning thing.
There are epoxies that are already very thin that work wonders for coating the engine compartment.
On the cheap is hobby store 20 minute finish epoxy.
More expensive and what I use, West Systems epoxy. You really have to be into it because West Systems is sold by the gallon.
To apply it I use cheap foam brushes I get from the dollar store (they are great for laying up fiberglass as well)