tape the outside of the fins? thin strips I assume would do it.

I never worried about it when I did mine...came out fine.  Keep coats thin,  give a minute or two for paint to tack i bit before hitting again.  Use heat paint,...I bought rattle can low gloss engine paint at Checker auto.

Lay engine on side, paint top edge? Let dry, lay on other side? But really, who is gonna be in a position to critique the area between the fins? Someone doesnt like it, tell em paint it..