I've tried various things over the years and I think the best way to do it is to rattle can paint the entire engine block. Then when you're ready to use it you just redo the hot tank and it is ready to go.

If you don't like that idea then you can try a spray on Cosmolene but my experience is that those sprays don't last as long as paint and it still requires a trip thru the hot tank to get them clean for assembly.