I would not attempt to use one to paint something large and flat like a hood on some sawhorses for example. No way would you get a nice even finish on something that large with a Preval. You could use one to apply a two part primer on a hood if you wanted as you would eventually sand it and paint it later with a real paint gun.

I use them for priming parts (big and small), and painting smallish things. I have painted engine blocks and heads with them and they came out with a nice even finish. Rough cast iron engine parts are very forgiving with any paint.

Doing an oil pan and valve covers works if you have the paint mixture aligned with the air temp so it flows out smooth. Too much reducer and you'll get runs. Not enough and its orange peel city - same as with a real paint gun.

They do leak if you tilt them too far back when the jar is full. The propellant will ice up and stop spraying if you hold the trigger down continuously. For painting an engine that should not be an issue.

https://youtu.be/bO_LCNHA5Vc