Hi Ian
I usually use a clear varnish made by "Humbrol". It is found in model shops and is part of the comprehensive range of paint colours and finishes used on Airfix models and other plastic construction kits. It is available in very small tins about 1" diameter and 1"deep. It is also available in slightly larger tins and quite small aerosol spray containers. I use this for transfers on headstock, mudguards, toolboxes and number plates (the spray is particularly useful on this last item). Petrol tanks need a completely different means of protecting transfers as most of the non-cellulose or non-two pack varnishes are affected by petrol. Chris.