Can you describe how it looks? If there is not enough texture you’re putting it on too thin. It needs to go on quite a bit heavier than regular paint. As it cures it shrinks and creates the wrinkles. But if there’s not enough film thickness it won’t be able to shrink enough. If the texture is uneven, work on your spray technique to maintain consistent speed and distance. If it’s cold where you are, I’d also avoid spraying if it’s below about 60F in the room where you’re spraying.

Sorry, I'm not quite sure what you're asking. Are you looking to spray your engine cover to match the body of your car? If it's a base/clear that is catalyzed (uses hardener), my understanding is it will take a fair amount of heat and would probably be OK anywhere besides exhaust manifolds.