When applying gloss polyurethane varnish to my latest project, I wound up with brush marks. What should I have done differently?

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Thinning each coat of varnish with mineral spirits allows the finish to flow out more smoothly, making brush marks less likely to remain as it dries.

Q.

When applying gloss polyurethane varnish to my latest project, I wound up with brush marks. I used a good brush designed for oil-based paints and stains; what should I have done differently?
—Joe Bonavita, Oxford, Conn.

A.

Joe, several factors contribute to a great-looking finish with standard varnish or polyurethane. For your next project, buy a China bristle brush with a tapered profile and flagged (split) ends. With this, you can lay down an even coat through a long brush stroke. Next, warm your workshop, if necessary, to at least 70°. Then, thin the varnish by mixing it 50/50 with mineral spirits. Thinning improves the flow-out and curing qualities of the critical first coat. Brush on a coat of varnish; hold the brush at a right angle 10° to the surface; and, working in the direction of the grain, lightly skim the varnish with the bristle tips to help level it.

Let the first coat dry for 24 hours, sand it lightly with 22D-grit sandpaper wrapped on a block, and remove the sanding dust with a cloth moistened with mineral spirits.

Examine the varnish for brush marks. If you see any, sand again with 220-grit paper, and remove the dust.

Now apply a second coat that has been thinned to 25% mineral spirits and 75% varnish, using the same brush technique as before. Allow the coat to dry, and sand with 320-grit sandpaper. Continue through two more coats with unthinned varnish.

Finally, go a bit further to create a perfectly level surface with a consistent sheen. Allow the final coal to cure for a week, and then use 6OO-grit silicon carbide sandpaper wrapped on a sanding block to level it. Rub with 0000 steel wool and paste wax for a satin sheen, or follow the 600-grit sandpaper with finer sandpaper or rubbing compounds to create a glossier look.