Most of the above advice concerns Mylar seals, so the real question is what solvent Uli needs to prep the surface for the cloth tape. You can try anything, but in order of aggressiveness (And possible damage to the PU paint) Methyl Ethyl Ketone is very aggressive and evaporates with little residue, acetone is second in both qualities and alcohol is the gentlest. It is also possible that the tape you are using does not have as good an adhesive as the original, or it needs a longer time to take a set. Let it cure for a while and use a roller to ensure the join is compressed as much as possible.