Hi, Mike
I use good old-fashioned Ronseal Hardglaze Polyurethane varnish. It thins with white spirits just fine. Having said that, they have recently changed the formula a touch to bring it down from 'Very High" volatiles to "High" volatiles and I'm not sure you can quite go to the same degree of thinning without getting other issues. Still OK to about 30%, though. I've also recently used it un-thinned (the new formula does seem to be a touch thinner) using a simple cheapish artist's fan brush, with pretty good results. This is the brush I used:
....and this was the result:
I really can't get on with acrylics sold in DIY outlets - they seem to stay soft and tacky. A lot of the commercial finishes tend to use multi-pack products or accelerated UV curing, etc.. Much of the available household stuff I find on the very edge of 'fit for purpose' at best.
There's a guy on one of the UK forums who gets spectacular results with wipe-on Chestnut Melamine, but it is far too fumy to use in the house...