Best way to keep the stains off the pipe is to not use that junk. There is no reason under the sun, to use a "primer/cleaner" on new pipe that has never been used. (meaning buried in the ground.) Code calls for primer/cleaner on 4" pipe or larger only (not called out for pipes smaller than that,) and primer does not make the glue joint any better, or any worse, than it would be if not used, unless the pipe is extremely old and/or dirty. The claims by plumbers (including so called "master" plumbers) and lay persons on this topic are complete nonsense, sponsored by the "glue manufacturers, because they could not sell primer/cleaner. Legislation for purposes of profit. Proper solvent welding technique produces leak free joins primer or not, poor solvent welding technique produces leaky joins, primer or not. Simple logic tells what is true here.