Had stripped my engine bay down to bare steel and hit the bare metal with a zinc phosphate metal prep liquid. I did too large of an area and it dried on and left a white, scummy looking accumulation of zinc phosphate on the metal. It's very tough to remove and I want to get back down to bare clean metal before applying epoxy primer. Any ideas on how to get this stuff off other than spending hours with the wire wheel? Any other kind of cleaner or product eat through this stuff?

Hit it with a red scotchbrite pad which will lightly scuff the surface. Then some wax and grease remover or Prep Sol type product.

Reading your post some more, usually the way metal prep is applied, you neutralize it with water and wipe it down when you are done. What does the surface look like now that it is hard to get off?

Will the Metal Etch remove the residue? I Like to wipe the Metal Etch off with Lacquer Thinner after it has cleaned the steel but before it begins leaving the coating...

Here's what it looks like. The product I used was por-15 brand metal ready. I know it works if you keep it wet, then clean it off after, but when I was doing it in the winter I guess my garage was warmer and drier than I thought and it ended up drying on before I cleaned it off. You can see the difference between where the residue is and where I cleaned it off with the wire wheel on the angle grinder. If the engine bay wasn't so much hard to reach areas and awkward corners I would just do the whole thing this way but it's a real PITA.

Laquer thinner touches it but doesn't get even close to all of it off. Neither does trying to re-wet it with more of the metal ready fluid help much either.

A good high free caustic cleaner, found in some oven cleaning products or chromic acid. The latter being the second choice as it is a carcinogen, and a last resort. It needs to be handled carefully. You can't buy it, but you can get the recipe online and purchase the ingredients online.

Some metal parts that cannot be tumbled are dipped in the chromic acid to remove zinc phosphate.

EDIT: Another option would be to make a cheap soda blaster and try and remove it that way. I made one and it works well for small jobs like what you got there. Here is a link to make it, cost is about ten bucks and whatever a tub of baking soda is.

http://www.aircooledtech.com/tools-on-the-cheap/soda_blaster/