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I've used Rockhard varnish; brushing it on, but with a bit different technique. Some background:
My absolute favorite guitar finish was Mowhawk's '4-hr Rubbing Varnish'. Sadly, they stopped making it a few years ago, after trying a formula change that was NOT satisfactory. I've been looking for a decent replacement since.
The best things about the 4-hr varnish were that it really did dry enough to recoat in four hours (even on BRW!), and that there was almost no problem with 'witness lines'. Those show up when the varnish hardens from the surface down. As you sand through subsequent coats you go from the softer part of the top coat into the hard surface of the one underneath, and there's a shiny line that shows the junction. The Rockhard varnish does show witness lines, but they can be minimised.
There are several tactics that can be used for this. One is to re-coat as soon as possible, so that the stuff underneath isn't too hard on the surface. Another is to add in a little acetone, which helps the new coat to 'bite'. The third is to avoid sanding the final coat if at all possible. The only way to do this is the get the varnish on really level right from the start, although you _can_ sand intermediate coats some. That's the object behind Sexauer's expensive brush; it allows him to spread the stuff out well. Then all you have to worry about (ALL!) is 'sand' (tiny bits of dried varnish from the brush), dust and suicidal bugs.
I thin the varnish out the way violin makers do, with about 25% mineral spirits, and 5% acetone. The acetone, aside from helping it 'bite', 'flashes off' fairly quickly, so that the coat thickens up and is less likely to run. A small amount (a few drops per quart) of kerosene (or spike oil, if you prefer the smell and can afford it) acts as a brushing agent, giving you more time to level the stuff out and helping it to self-level some. I use a common bristle brush, 3-1/2" wide, and _never_ dip more than 1/4 of the bristles into the varnish.
With care, you can get nice level thin coats on this way. The first one takes as much as 12 hours to dry, but after that it will be dry enough to re-coat in four hours. You _must_ recoat it as soon as it's dry enough, but not a moment earlier, to minimise witness lines. If you try to hit it while it's still 'gamey' the brush will stick, and you'll end up with a mess.
With a good, thin, French polish type fill, even on the spruce, I can get a nice finish this way with 5-6 coats, and they are very thin. Sand back each coat lightly with Micromesh, 1800 grit or so, just to knock down the 'sand' and dust. After the first coat do two in a day, or even three if you want to.
Allow the whole thing to settle for a couple of days after the last coat, and then sand back as little as possible. The idea is to only sand into the last few coats you put on, which followed each other closely enough to bond well. If you get witness lines, sand back to level and put on a couple more coats.
It would probably help a lot to wait a month or more to do the final polish. This will allow the film to harden more uniformly, and reduce the 'witness' problem to the minimum.
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