My ingredients are Feast Watson spar varnish, Penetrol and real gum turpentine to help the stuff flow, and a proper varnish brush, which is wide and very thin, so it doesn't hold too much varnish, with super smooth bristles, so it doesn't leave great big ugly brush marks. I'm thinning the varnish out with ~15 percent penetrol and a further 5 odd percent turpentine. That gives me a mix that flows out nicely. Of course that's a recipe that's highly dependent on environment, brush, technique...
I started sanding with 180 grit, but found 400 works better in the latest coats. Here's the rowing thwart thus far. There's a bit of general lumpiness but the gloss level I'm getting is fairly good:
My rowlock bases and tabernacle have had a couple fewer coats. In the case of the rowlock base that doesn't seem to be an issue, but the tabernacle still needs some love.
I find when I'm putting it on it's good to keep a little container with mixed varnish+penetrol+turps, plus one with half an inch of pure turps in it handy, so I can thin out and clean the brush periodically, to keep it from sticking to everything.