I did my engine bay with a brush, thinner, oil based paints, etc... came out ok, and not black.
Also for the inside rails, I sanded the inner part of the fender with 600grit and then brushed a clear coat over it, cleaned with #0000 steel wool, and then polished. It looks almost identical to the newer paint outside the car.
Maybe you can do the underside by hand, and the harder to reach areas, then shoot the rest.
