5 ideal applications for sanding sealer
Save time and trouble
If you routinely apply sanding sealer before finishing, you may be taking an extra, unnecessary step. Truth is, any film-forming finish works as a "sealer" because it closes off the pores of the wood and lays a base for the topcoats. So most of the time, you can seal wood using the same finish you'll use for additional coats.
Some situations, however, benefit from a sealer. For example, when refinishing furniture with lacquer or polyurethane, silicone on the surface can cause the new finish to pull away from the contaminated spots—a condition called fisheye. In this case, applying a thin sealer coat of shellac (which isn't affected by silicone) separates the silicone contaminant from the topcoats.
Following are five more ways sanding sealer can save you time and trouble.
Seal knots for a better bond
Resins in pine and the oily surfaces of some exotic woods, such as teak, prevent film finishes, such as polyurethane, from bonding with the wood. When that happens, the top coat can separate as it did on the strip of tape shown. However, a thin coat of shellac, like the one on slide one, seals in resins and oils to provide better grip for topcoats. (To choose the appropriate shellac, see the Shop Tip on the last slide.)
Stop the pain of raised grain
Water-based finishes tend to raise wood fibers more than oil-based finishes, especially on red oak. To minimize grain-raising and make the first coat easier to sand, apply a water-based sanding sealer before spraying or brushing on topcoats.
Finish in a flash with a fast-drying sealer
Instead of waiting a day between a sealer coat of poly and the first topcoat, speed up the process by first applying a fast-drying sanding sealer. This coat of sanding sealer, for example, dried hard enough to sand in an hour, giving us a jump on the first topcoat.
Seal pores before filling them
Filling pores helps you achieve an ultra-smooth surface for a high-gloss finish, but tinted pore fillers also color the wood surface. Before applying pore filler, first seal the entire surface using a sanding sealer and lightly sand off any raised grain. Now the excess pore filler will wipe away cleanly, staying only in the pores, without discoloring the wood.
Keep dye from going bye-bye
Water-based dye provides a deep color, but unlike stain, it lacks binder to seal it in place. So when you top it with water-based finish, the dye can redissolve and contaminate the topcoat. To lock it in place, apply a thin coat of shellac over the dye. The alcohol in the shellac won't redissolve the dye.
Select a dewaxed shellac for strong topcoat bonding
Although shellac can be used as a sealer under almost any other finish, not all shellac works well as a sealer. Unless sold as "dewaxed," shellac can leave behind a surface too slippery to bond with non-shellac finishes—especially polyurethane.
Before you buy premixed shellac, check the label (see example) to see if the contents can be used beneath other finishes. Or make your own shellac sealer by dissolving 2 oz of dewaxed shellac flakes in 16 oz of denatured alcohol (a 1-lb cut).