Wood floors often need cleaned before refinish
Q. I'm refinishing the stairs in my turn-of-the-century home, and when I pulled off the carpet I found this black stuff underneath. I don't think it's paint. What could it be?
A. This could be one of two things. It could be shellac, which would be easily removed using alcohol. But the black substance is probably a varnish that was popular to use on wood in the era when your house was built. This varnish is thin and doesn't penetrate the wood, and will be easily cleaned with a mild paint remover.
Q. What is meant by the term "grade of abrasive paper" and what is meant by the term "raising the grain?"
A. The "grade" (or grit) of abrasive paper refers to the size of the abrasive particles in the sandpaper. Given the same number of passes and the same amount of pressure, paper with larger particles sands deeper and rougher than paper containing smaller particles. A lower number indicates that the grade of the paper is used for rough sanding where a high number indicates the sandpaper is meant for finish sanding.
Generally speaking, 30- and 60-grit papers are used for rough sanding, 100- to 150-grit sandpaper is for medium sanding and 220-grit sandpaper is used for finish sanding. But this changes with the type of wood and whether the sanding is done by hand or with a machine. The best way to determine what grit to use is to test-sand. Start with the finer grits (150 to 220) and slowly work up to the rougher grades.
Sanding a soft wood with rough sandpaper, or sanding perpendicular to the wood fibers, could possibly tear the wood fibers -- causing the grain to raise to the surface. Another way of causing the grain to raise is to over-wet wood; it's what painters must contend with after the first coat of paint is applied. At this point, and once the paint or varnish has dried, the first coat and the raised surface must be smoothed. The second coat of finish usually will not raise the grain because the wood is protected from absorbing moisture by the previous coat.