What are you guys using to paint iron blocks and heads for I/Os? Something that will hold up to the constant humidity of being stored in a slip.

Prep is everything, you can't have a single bit of grease or lube anywhere on the block. Tape off the mating surfaces. Some people want their casting plugs painted, some don't. I don't so put a layer of grease in there and paint right over and wipe grease out, shiny plugs!

I've used Rust-Oleum engine primer and your selection of engine paint. Get the primer on there good and don't be afraid of 3+ coats

I soaked the heads in simple green and let soak. Then pressure washed them. Put them into an electrolysis tank and got the carbon and some minor rust off. Then pressure washed again. I'd say they're about as clean as they'll ever get. So this should work out well. Thank you.

Rustoleum industrial enamel,...

As mentioned, prep is key, but it's more forgiving than painting an aluminum outdrive.

Lots of good engine paints...look for one that can re-coat in 10 minutes, and dries to touch in 60 min. They mean dry to touch.

Lots of coats works, but don't overdo it. Thick paint will chip off easier etc.

Same as chrome plating too thick. It makes it brittle, and causes issues.

I’m going to use 545 primer and awlgrip. I know it’s not listed for heat but the fishing guys are using it on diesels and liking it.

I remembered an old thread on engine paint that included Eddie Young. Some good info. Took some searching but here it is
custom engine paint?

Padraig

Once I get them as clean as I think they can be, I take a handheld propane torch and 'flame' the surfaces. You can see it go a flat color. The casting is so porous that minute traces of oil won't come out. The flame and the heat burn it off! Let it cool a bit, then primer before moisture creeps back. Never had one peel!

Eddie Young took an entire day to paint both motors.
Lots of steps thats for sure.