hHow clean is clean? I have scraped it with a a scraper, wire brushed the residue left sticking to the joint. There is still a thin layer of sticky black residue left from the wire brushing on parts....
What tape are you using? If it is Eternabond they recommend cleaning with Acetone. "For EPDM and metal clean with a "hot" (fast evaporating) solvent like - lacquer thinner, acetone, automotive brake parts cleaner, or a commercial EPDM cleaner, or a good, non-residue leaving cleaner."