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IMO it all depends on the firewall, inner fenderwells....here is an fellow FTE members truck and what he used on the inner fenderwells.
I used Rustoleum Industrial Aluminum spray paint, comes in a 33% more can. Used less than half can to do inner fenders for around $12 a can, the primer is about $8. It dries really fast, goes on thick, and never gets runs. I used Rustoleum Industrial spray paint for everything. I have painted all kinds of things like the fire walls on a 68 cougar xr7, 1956 ford, 1955 ford Crown Victoria, numerous parts, and a whole van for a friend.
It goes on really good with one coat, lots of coverage, 2nd coat has to be within an hour, or after 48 or something, I just do 1 coat. I tried other spray paint, but this is my favorite, for a rattle can job anyways. It's enamel, so it takes long time to cure. It does dull, and in a year I do expect to see rust coming out again.
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