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I recently suffered from an access of brain power, and thought to actually save a tutorial I wrote up about using Rockhard varnish. The only thing I left out, I think, is cleaning the brush, which I do with Ivory soap and water, after getting as much varnish out as possible with thinner and drying the brush thoroughly. Anyway, here it is...
The product I'm using is Behlen's 'Rock Hard Table Top Varnish'. As with any finish, it takes a while to get the hang of it, but you can get excellent results. Once mastered (do we ever 'master' a finish technique?) it probably does not take more man-hours than French polish, but the lead time is longer.
As with all of the modern synthetic-resin varnishes I know of, this one has some issues that make it hard to use. Aside from the usual ones that you get with all brushed finishes, such as 'sand' and runs, you can get problems with 'witness lines'. There are also circumstances when it will not dry as fast as it should.
'Sand' is what they call the little particles of varnish that you just can't get out of the brush, which produce a nubbly surface when it dries. All you can do is get that down to a manegeable level by cleaning the brush really well. Then you have to sand off the nibs, and that's when the problems start.
This varnish hardens from the top down. As you sand through the coat on top you go from a soft layer into a harder one, and this shows up as a shiny 'witness line'. The only ways to get rid of it are to avoid the problem in the first place, by not sanding through, or to allow the finish to cure fully enough through the film thickness that the hardness is substantially the same all the way down.
Avoiding sand-through in the first place is a matter of getting each new coat perfectly level and smooth (except for the sand). I find this easier to do with very thin coats. However, if you just add a lot of their proprietary reducer (and DON'T use hardware store thinner for this!) you'll end up with something that will stay fluid enough to run at the slightest provocation, and there goes your uniform smooth coat.
I find that thinning with about 25% reducer and 5% acetone works well. The acetone 'flashes off' fairly quickly, giving you enough time to spread the varnish in a thin coat before the brush starts to grab, but leaving the film viscous enough to avoid runs and sags if you're careful. The acetone also seems to help the new coat to 'bite' into the one below. Putting is a few drops of kerosene per pint of mix provides a little lubrication for the brush, and does not seem to make problems.
Applying oil varnish is something of an art in itself. The stuff starts to set up as soon as it hits the surface, and if you brush over it once it has started to gel, it will look like a plowed field. Thus you have to work off a wet edge.
I use a small soft brush these days: a camel hair 'wash' brush 3/4" or 1" wide. Dip the brush in the varnish, so that it's wet only about half way up the bristles. Dab it onto the back in the upper bout, making two patches about 2" long along the grain up near the top edge. Then use the brush to spread these across the surface, trying to get complete coverage. Always brush OFF the edge, never onto it, as that will squeeze varnish from the brush onto the sides and make a run. When you've got the surface pretty well covered, brush lightly along the grain with overlapping strokes, feathering the varnish out at the lower edge of the wet patch and making the surface as smooth as you can. Then pick up another load, and start dabbing it on right at the wet edge. When you're doing the sides make sure you don't make runs on the back edges: check after every brush stroke, and brush out any runs very lightly, moving the brush slowly so it doesn't mess up the gelled varnish on the back. Ditto when you're doing the top. I do the back and sides while holding the guitar with my hand through the soundhole, then do the top starting from the lower edge. This assumes the neck is not on; if it is it's a lot easier!
Varnishes harden by a chemical reaction that involves oxidation of the drying oils. Some woods, notably tropical ones, can contain non-drying oils that will leach up into the first coat of varnish, and slow the hardening. I've had the varnish stay tacky for a week or so on Macassar ebony bindings. You cannot successfully put on the second coat until the first one has fully set up. It might skin over, but the varnish underneath is still soft, and the whole film will come off. Looks bad.
UV light speeds up the drying process. I'm not sure if it's the light directly, or the ozone it produces, but even a bit of UV can be a big help with that first coat if there's a problem.
Depending on how things are going I might wet sand the surface at some point. If there are sags or runs that's the only way to get it level for the final coats. If you sand through someplace you'll need 5 coats or so of this thin varnish to build up enough to polish back.
Once the first coat has set up, it should dry well enough to 'dust up' when sanded in four hours or so. With luck you can get two or three coats on in a day. I always try to do the final two coats as close together in time as I can, so that the surface of the penultimate one is not too hard. This is a little tricky: a slightly-less-than-cured coat will tend to drag the brush a bit, and make it a little harder to get the next one really level. At least do them both on the same day, rather than letting the next-to-last coat harden over night.
Once you have built up a thick enough coating (I use anywhere from 5-8 coats, or even more) you have to wait. Then you need to wait again. Varnish shrinks a lot as it cures, and it takes as much as three months to cure 'fully' ( some aver that it never does, but in three months it will be about as cured as it's going to get). Meanwhile, as you wait, you'll find that the great fill job you did wasn't all that great. Surface prep here is as important as it is with French polish, but you knew that. In practice, two weeks is usually sufficient after you've gotten the last coat on to allow for rubbing out without too much risk of witness lines, but three weeks is even better.
If you were a good boy and ate your veggies you'll have a smooth shiny coating except for the sand and a brush hair or two. I sand things out with the finest grit of Micromesh that will work in a realistic time: try 3200, and go to 2400 if you have to. Once you have gotten the bulk of the sand out, go to the next grits in the usual way.
When I've gotten to #3600 or 4000, I use plastic polish. 'Novus' brand works well: it's not as aggressive as Meguier's and leaves a better surface on a finish that is not as hard as nitro. You can then go to white buffing compound on a wheel for the final shine. If the plastic polish shows witness lines, allow the whole thing to sit for a week or two, and then buff it. I did that with the guitars I took to Montreal last year: they were OK for the show, but looked that much better a week afterward when they were buffed out. Darn.
This varnish is hard enough to take a really nice shine. It's tougher than either nitro or shellac, IMO. I end up with more finish on the surface this way than I would with FP, but not all that much more, and it's a more protective film, with better chemical and abrasion resistance. I've used it on the tops of steel strings with no evident harm to the tone, but I've been too chicken to try it on a classical top yet. I don't doubt that it adds more mass and damping the FP.
Oh yeah; filler. So far I've used varnish fill (that is, no fill), a French polish pumice fill, water based filler, cyano, and epoxy, and they all work so long as you sand back well to the wood.
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