Dear Fern, well we are basically speaking about solvent based varnish and their recipes are probably really similar.
In States you can find Spike It products, in Europe we have (now) the Finnish CAB Coat varnishes or those ones by the Belgian Brightbaits ... and all of them are not really cheap, but they don't need a top coating.
Solvents like MEK, THF or Acetone, in my opinion, snap off the poured plastisol surface and permit to the pigment to be mixed with PVC, instead Cyclohexanone should be a retarder of this kind of varnish, good for airbrushing tecnique.
My hobbyst process is really simple: cleaning of soft bait surface with a solvent like MEK, "painting" of bait surface with powdered pigments (pearls, metallic, shifting) using a sponge, heating the surface with a torch, top coating with the cold mix 50 PVC glue/50 solvent. The solvent is a temporarily thinner of PVC glue.
If there is someone that can suggest any kind of adjustment/improvement, I will really appreciate it.
I lastly bought some thin Ukrainian cheap varnishes, they work great with a brush and it's possible to thin them with acetone ... also in this solvent base varnish a top coat isn't needed.
Bye Cami