Before I painted my plenum, IH's and valve covers I had the usual discoloration of the factory paint. I was able to get the blotchiness and stains out by using a very light application acetone on a clean rag. You have to keep in mind that acetone will attack the paint. So it is very important to use a minimum of acetone on the rag and wipe across the paint lightly. You will find that it takes off the surface oxidation and grime. You will be able to even out the painted surface very nicely. Again, use too much and/or rub too hard and you'll go through the paint.
Good luck...
H
