I'm in the process of sealing the engine bay now with epoxy... you mention thinning it down which I hadn't planned on. Do you just thin it with Methalated Spirits
The simplest way to thin epoxy is to warm it. A heat gun, or even a hair dryer, will have it flowing like water in a few seconds. This does make it 'set' faster so use 30 minute (45 if you have it) epoxy.
Methylated Spirits will work. You need very little.
Acetone, if you have some, is an epoxy thiner. You need very little.
Carrell