Here’s some information I posted on another thread, essentially:- MJA also recommends an intermediary coat of liquin before any varnish is applied.
After weighing up the pros and cons and taking full responsibility for any problems that might arise, with a 2" brush as similar as I had to the kind Anthony suggested, I applied a thin coat of liquin original to my Vermeer. I was content that I wasn’t going to be putting any more paint on there and may yet add an actual real layer of varnish on top just to try that out, but so far I’m very happy with the effect as it stands. It has a satin finish and has unified the sheen and given an equal saturation of darks and lights across the painting.
I know it is often recommended to wait at least 6-12 months before varnishing so that layers of paint have time to fully cure, but in practice these things often need to be done sooner. Mine was touch dry and left an additional two weeks before I did anything (MJA said this was an acceptable timeframe and his students often varnished paintings much sooner than that out of necessity). I had also used alkyd paints and mixed a bit of liquin in with the medium to aid drying. Of course if there was a lot of oil added for some reason or some thick impasto I would be more cautious.