Polyurethane is a common varnish for woodworking projects, as it is a transparent compound that puts a hard layer atop whatever other finish is already in place (if any), protecting the wood without seriously altering its appearance. However, applying polyurethane to an already existing layer of varnish, such as old polyurethane, shellac or lacquer, runs the risk of starting a nasty reaction that might compromise both the old varnish and the new layer of polyurethane.
List of Items Needed
- Clean cloth
- Denatured alcohol, mineral spirits or other cleaning preparatory solution
- Hand sanding block
- Fine-grit sandpaper
- Tack cloth
- Paint brush(es)
- Razor blade
Study the wood object to determine what kind of varnish was used, and choose your polyurethane accordingly. Lacquer, for example, demands the use of oil-based polyurethane, since acrylic polyurethane will react with the lacquer.
Wipe down the surface of the wood object to clean it prior to sealing. Once again, choose your cleaning agent based on the varnish already in use. Denatured alcohol is a good preparatory solution for most applications, excepting shellac, which is soluble in alcohol.
Scuff up the surface with a hand sanding block and fine,180- to 240-grit sandpaper. Sand lightly and with the wood grain, if the grain is visible. Covering the surface of the old varnish with mirco-scratches will provide a better bond for the polyurethane.
Wipe down the object again with a clean tack cloth, removing any sawdust or debris.
Apply a coat of polyurethane, using long, even strokes of the brush. Allow this coat to dry as directed by the manufacturer, eliminate any solidified droplets that form on the surface by shaving them off with a razor blade, and then apply a second coat. For a wood object that sees heavy physical contact (such as a step stool), you may wish to add a third coat.