I have found that, especially on "old" heavy grime, spraying it down with WD40 first, then a little elbow grease, then hosing it down with Gunk, then some more elbow grease works the best for me. Be aware of your surroundings when you do this or you may have your neighbors, EPA, etc... on your butt as that Gunk can really stink for days. I try to use the local do it yourself carwash place when possible. Then I don't have to worry about cleanup. I never paint right afterwards, I always use some laquer thinner or denatured alcohol, etc... and give it a good scrub down. Prep it like you would anything else before painting and you shouldn't have any problems.
