Car is together, have to keep it somewhat mobile where i live. I was thinking of using a Dremel with a small wire wheel and my hand, use some vinegar on rags to get the harder spots, then some por 15 or the like. No holes just some surface scale. Still have not figured how to get those cursed wiper arms off and i have the Service manual!!

Power washers if wand end is small enough. Also have used assorted siphon type sprayers to clean out those nasty corners. Some of those longer engine cleaning brushes that are flexible. Most mess will drain away at end openings. Not a fun area to clean for sure.