I actually have used the red oxide Rustoleum rusty metal primer from a rattle can on cleaned blocks and have had excellent results over the years. All the blocks I have done come from the machinest bake and blast machine and then I wash them with hot soapy water and dried for at least a couple hours.

I just do it to help keep any flash over or surface rust from forming during the build and help seal the pores in the metal. Sometimes I do the whole block, other times it's just the lifter valley timing chain area. It's just a pain to mask when you spray, but I like the results.

383 build:





440 build:





Brand X build: