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Finishing Tips by Bob Flexner: Sanding

Jul 6, 2016 | Expert's Corner | 0 comments

When sanding wood in preparation for a stain or finish, you need to remove all the problems in the  wood – mill marks, tear outs, gouges, etc. – with the coarsest grit sandpaper you’re using before  moving on to finer grits (to remove the coarse-grit scratches). This means that the coarse-grit  sandpaper you begin with should be able to remove the problems quickly and efficiently to reduce  the amount of work required.

On the other hand, with factory pre-sanded veneered plywood or mdf, beginning with 150-grit sandpaper is usually adequate.
As an example, 100- or 120-grit sandpaper is usually coarse enough to begin with. Beginning with 150-grit sandpaper is usually inefficient because you have to sand too long to remove the problems.

These are general observations. We all sand differently, so you may want to adjust the beginning grit you sand with depending on the problems you want to remove. But the basic rule still applies: It’s most efficient to remove all the problems with the coarsest grit you’re using before moving to finer grits.

Testing Finishes for Heat Resistance  

Resistance to damage from hot objects is an important finish quality for tabletops and counter surfaces in kitchen and dining areas. The type of finish you’re using will be a strong clue to its resistance. For example, oil-based polyurethane and catalyzed lacquer are very resistant to heat damage, while shellac, lacquer and water-based finishes are less so.

There are two tests for heat resistance that are easy to do.

To test for dry heat resistance, place a metal cup or pot containing water heated to just below boiling on a fully cured finish, as shown in the accompanying picture. Remove the cooled container after an hour and look for splits,  indentations or discoloration in the finish.

 To test for wet heat resistance, do the same as for dry heat resistance,  but place a cotton cloth or cheesecloth wetted with the same hot water  under the cup or pot. After an hour check the surface for splits in the  finish or discoloration.

The easy way to do this is with architect’s pencils because these vary in hardness themselves.
Finishes vary in hardness and scratch resistance. To a large extent the  relative hardness of various finishes are known. For example, oil-based  polyurethane is more scratch resistant than lacquer or shellac. So are catalyzed finishes and water-based finishes. But what if you want to be  more exact about the differences, or want to compare brands within any single type of finish?

Begin by buying a set of architect’s drawing pencils ranging in hardness from about 2B (soft) to 5H (hard). Sharpen each pencil using a knife so you leave the sharp cylindrical edge of the lead intact. If you damage this edge, or if it becomes worn, sand it flat, holding the pencil 90 degrees to the sandpaper.

Beginning with one of the softer pencils, hold it as you would for writing and push it forward across the fully cured finish as shown in the accompanying picture. Maintaining equal pressure, follow with pencils of increasing hardness until you find one that cuts into the finish.

The hardness rating for that finish is the number of the previous pencil – the hardest lead that doesn’t cut.