MP (after PP) is needed on some paints and not on others. The better you break down the PP the less chance you'll need the MP but you can't generalize.
I'm the "Autopina Heretic" once again when it comes to claying and LSPs. IMO (based on many years of spot-claying with every wash) it's often possible to clay without removing an appreciable amount of LSP. Heh heh, seriously, I'd notice if I were wearing away the LSP on the areas that get clayed all the time (I use clay on bugs and tar). Whenever I've *tried* to "cut through" or "wear/clean away" my LSP with clay I had to be exceptionally aggressive about it, to the point that I felt I was claying incorrectly. It did eventually take me down to paint, but it was a weird experience...I just *never* clay that aggressively. I use the clay to pick contaminants out of my LSP and that doesn't take much.
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