I coated inside and out of my 69 Vette pan and my 69 Camaro with truck bed liner. I thinned it down a bit to avoid excess build up where it was not needed. I shot the inner firewall and the ceiling, inside the trunk and backside of the quarter panels.
Cost?
You can get the gun for about $20 at most paint supply stores (body shultz gun). There are different types of "rhino" liner (epoxy, urothane, etc) and brands.
It usually runs around $100 or so for a two quart kit. You want to get the two part kind as it bonds better and should last much longer.
