I find Oil Flo Safety Solvent and Wurth Citrus Degreaser the 2 best
products for removing cosmoline (Zymol Strik is a little too costly and no
more effective).

For first pass, I let the product sit on the surface for 15-20 minutes (I
typically use the Oil Flo as it can be purchased in more economical 1
gallon containers) and wash the surface with a 800-1000psi wide spray from
a power washer (if you're fortunate enough to have a hot water washer, set
the temp to around 180F, higher if there are no plastics in the area you're
cleaning). This should remove most of the heavy paraffin like debris. For
second pass, I'll use another shot of cleaner and some very fine (0000)
steel wool to loosen the rest of the coating. Rinse again with water.
Final pass with cleaner (this time either P21 Total Wash or Wurth Degreaser
cut 2:1) and a cloth rag. Final rinse and dry. I find this procedure
effective.

This stuff took 15 year old cosmoline off my 528e's intake manifold with
nothing more than an old toothbrush and some water.
http://www.greased-lightning.com

Anyway, the only thing I have to add to the process was the removal of
cosmoline (and all the other crap which can adhere to the bottom of a car).
I used PPG DX 330 Wax and Grease Remover. This did the trick quite easily
and is safe for painted surfaces. I had occasionally used to use it to
remove really tough tar from my cars although clay has made this use
obsolete for me. That is why I had some DX 330 handy. I found DX 330 is
great for removing cosmoline although I am not sure if it would do anything
for baked-on cosmoline. It can be purchased from any paint store, NAPA,
etc. There is a similar product available from Ditzlers called Prep Solv I
believe as well. In addition, PPG has another product called DX 440 which
removes just about everything (wax, grease, tar, paint, etc.) right down to
the sheet metal I believe. Not good for finished surfaces although I have
never used DX 440.