I've painted one or two engines. We didn't use primer. Just cleaned everything realy good and used high temp engine paint from discount auto parts. Fill a sink with hot water and let the cans float in it for 10 mins or so, occasianly rolling it around to make sure all the paint gets warm. Makes the paint go on rely nice and look rely thick and glossy.
i degreased my block with some auto cleaners the power washed the block the put a thick coat of heat paint on , but you can just go to you local parts store and pick up some engine paint.
You just have to make sur ethe block is clean and oil free and use engine paint. Its amazing what a can of engine paint can do to improve the look of a motor.
I just painted my block. The cleaning and sanding is a ******. I sanded everything with a wire brush until the block was silver in color. Also had to chip some rust layers with a screw driver and hammer. Anyway I have put in many, many hours cleaning and painting but in the end it's worth it. I used some Dupli-Color high temp paint. Oh and buy a mask, I forgot that (stupid me) and have been picking out nothing but black and blue snots ever since.
I had my block tanked before painting it. That ensures that the paint will stick, and look good for years.
I painted it ford engine gray. Nothing special, but along with some other cleaning, my engine bay looks descent I think.
Trust me, you won't stop at the block.