The rattle cans work well for an engine bay because once everything is back in, you don't see a large percentage of it. A high quality mixed spray paint in a can with a good nozzle can lay down paint surprisingly well, unlike a hardware store can. Then a 2k clear will make it all purty, stain resistant, and hide small flaws. I thought about going the gun/compressor route as I have a 60 gallon one with plenty of CFM. I would certainly use it for exterior or large panels as Scott mentions but for under the hood I didn't want to mess with measuring paint, mixing, removing all water from the lines, cleaning everything etc etc for each coat. Not to mention the $$$$. If I ran out of paint or messed up like when I put the wrong honey comb fender insulation on, I'd just order a couple more cans from the supplier and redo it. They got to know me pretty well by the end of it.