I had the same trouble....
I had the same trouble as your mentioning. I can give you some tips though.
Do not skip on the compounding. Any imperfections have to be taken care of during this phase.
I had to go over each inch of my trailer over 10 times with the compounder to get all the filform corrrosion out. Hopfully you don't have to go through that much trouble.
On to the cyclo.
Make sure the surface is clean and dry and the ambient temperature is between 65F and 80F. Use a cyclo wrap and put a very small dab of F7 on each head with a butter knife. Spread a very thin amount on.
Go over a two foot square area very slowly, about a foot very two seconds or so. Important to go each direction. Up to down, down to up, left to right, right to left, and each way diagonally. Keep going until a majority of the blackness is gone.
After you get this section done, move the wrap and go over another two foot area with the F7.
When your done with the amount you want to get done, or finish all the postions on the cloth, use the mineral spirits and mircofiber cloths to clean off all the residue of the F7.
Use a fresh cyclo wrap and go over the same areas with S using the same method. Up/down, down/up, left/right, right/left, diagonal...
When finished, use the microfiber cloth and mineral spirits to remove the S residue. I know perfect polish now says not to do this, so it's up to you. Their feeling is to keep the protection S offers you cannot use the mineral spirits on it.
Now take a new cyclo wrap and go over the area dry to get the last residue off and make it shine.
The next step is something I have started doing with great results. I got the idea from another forum member, Herb Spies.
Go over the trailer with NuFinish auto polish/wax. It has worked great so far keeping the rain water spots away.
My trailer still has some scratches from the compounding and each time it is gone over with the cyclo, it improves. Since I have to go over it once a year anyway, I'm just going to let the maintenance squeeze out that perfect shine as the years add up.
Good luck, and post some pictures!