Does anybody apply anything to the engine block to smooth out the rough areas before painting?
Only if you're going for over-restoration.
Steve is right. Casting marks and seams as well as some pitting showed on the cars. Grinding them off or filling them in is over restoration and not original.
If the block is real bad, you will want to smooth it down with some 80-100 grit paper, but by no means do you want to remove all the casting sand dimples. You will do a lot of extra work and the results won't be authentic, as has been stated.
Norman, I have a spare block for my 1912 van which has had a weld repair that required grinding. When I primed the repair I had to chuck a handful of sawdust into the wet paint to get the repair rough again.
Allan from down under.
Same thinking, as shared here, applies to chassis components as well. I can't stand a gleaming flawless front axle. (but that's my taste, or lack of same)
Heavy rust pits are a different story however as those are not an original texture.
Also, the more filler, primer and paint you put on the engine the hotter it will run. Keep it thin and don't prime it. Ford used a thin gilsonite slush and slathered it on with a brush.
Real thin paint (slushing primer it was called) used on the early engines....
didn't last long one can determine