I used to have a similar issue until I asked for help on RMWeb, I now either apply gloss varnish or klears floor polish to make the side shined, apply the transfers, paint on some decal fix and then apply a light dusting of testers dull coat, the transfer backing is then totally transparent... this method works brilliant for Rail Tec transfers and the backing always disappears, I normally leave decal fix on for 30 - 60 seconds to not melt it too much etc
I recently did a respray of a 47802 and used R3spray gloss DRS blue to avoid the need of applying gloss varnish.