You problem was the polyurethane, not the solvent. Unless you get a specific thinner, as recommended by the manufacturer, you are likely to be wasting your time, as the stuff has a high level of VOC's and, as you discovered, sets up like a rock. It makes a wonderful bulletproof surface but to call this stuff varnish is a misnomer. All it has in common with varnish is that it is a slightly amber, transparent surface that can be applied with a brush. Once you get a real varnish, get a real brush, china bristle for preference.

By the way, a paper mask will not do squat for you. Use an industrial face mask. If you must use that nasty stuff, work outdoors. Even so, the VOC's are likely to kill a few brain cells.

If you varnish, go for the high tung oil types, which are easier to touch up or refinish. If properly applied, they will outlast you.