Assuming paintwork is enamel, T-Cut would probably be your best bet here: small amount worked gently over the transfer area with the side of a blunt cocktail stick/wooden cuticle thingmy. Once transfers are gone, remove any remaining T-Cut with cotton buds; hey presto, nice high gloss area ready to receive new transfers. Just be careful not to work the T-Cut over any transfers etc., you wish to keep. Pic shows a renumber done exactly as above, after subsequent clear coat (deliberately uneven) and before weathering