You could try the new Humbrol Clear.
It says on the bottle that its a gloss varnish however it's a very thin sealant used by plastic aircraft and AFV modellers to seal their paint finishes in preparation for applying the decals. The benefit of this liquid is that the more you apply the glossier the finish so you can control how matt or glossy you want your model to be. It seals in the first coat and can be applied either by airbrush or by paint brush. It's apparently based upon the Johnson's Future Floor Polish liquid which was once available in abundance but the recipe changed two years ago which upset its qualities for the plastic modeller. I use it all the time on my AFV and aircraft models, it also prevents clear injection moulded plastics from fogging when using superglue or normal poly cement.
