While I've never had anyone else repaint my cars.I used to work for Earl Schieb's many years ago doing the body prep work and tapeing the cars.I don't know if they've changed at all.But it used to be all jobs were done with Earl's own paint.The paint had a slightly rough texture,similar to rock guard coating sprayed on the rocker panels of some cars.Maaco does offer paint other then their own at extra cost.But the bottom line is that every shop has different painters.Some are very good at their jobs.And some shouldn't have graduated from finger painting.Look at a finnished car before you agree to letting them do you'rs.As for diesel smoke,and oil smoke stains on the paint.There are cleaners availabe at auto parts stores that you can use yourself.And for extreemly bad and stubborn staining,there are tar and tree sap cleaners on the shelves for the really bad stuff.After cleaning,any good car polish will help keep you'r car cleaner.