Not here to disregard morbid goat, but you do not have to primer surfacer the whole engine compartment if your paint is not peeling, has alot of rust etc. There are as many ways to tackle this project as their are products out there Ben, and a lot of money can be wasted here if you are not careful. I will give you my car as an example...super clean rust free engine compartment...paint in pretty good condition and could be scuffed and sprayed, BUT, I am having the whole car media blasted to bare metal and epoxied prior to primer surfacer being applied. If I was not going that route and wanted to do an economical job relatively speaking, I would do just as I had previously described. Clean the engine bay, sand with 320/400, etch prime any bare metal I have present and apply surfacer to the areas I have sanded through. Allow to cure, and scuff the primered areas with 320/400, blow it down, clean with a mild wax and grease remover, mask the area and spray...Either method will produce the same end result IF you follow the paint manufacturers instructions to a 'T'.
I have over 30 years experience in autobody, in fact I teach collision repair and refinishing and I don't feel comfortable handing out advice all the time due to the varying circumstances that can arise at anytime during application/preparation. It is not due to my lack of knowledge, it's the fact that I cannot physically see your situation. Also the fact that materials are a small fortune these days makes me cautious, an example here would be a quart of reduced primer to spray the whole compartment 2X as compared to spot priming, maybe 4-6 oz. I will always try and give the most up to date correct info here, but ultimately you are on your own...see if you can track down a body tech/painter that can give you advice on materials/procedures, the case of beer it takes to lure them into your garage will be well worth it!
Good luck...J
