Nothing ventured,nothing gained.
Some good suggestions but also a bit dodgy over oils as most of the humbrol stuff is solvent based and will lift the finish if applied over oil with a brush .
Many moons ago I used to use a water based varnish from rowney , "no longer available " switched to GW matt varnish but in there infinite wisdom they stopped making it . also used to use letraset fixative also gone
None of this helps I know ,see you on Sunday