Q: Our white bathtub, no longer glazed, has developed a yellowish ring at the water line. The ring resists scouring powder, bleach and acid. The piping is copper and we have a glass-lined hot water tank. Do you know what this ring could be and how we can restore the tub's whiteness?

A: The yellowish stain could be from a number of things -- rust in pipes or oxidation from copper, or just an accumulation of soap curds. You say that the tub is no longer glazed. I assume this means it has a dull surface, and when porcelain loses its glaze it is somewhat porous, so stains will soak in and be hard to remove. I would suggest first trying to scrub with a product called Zud (sold in hardware stores and plumbing supply houses). If this doesn't work, make a paste by mixing liquid bleach with powdered whiting (chalk dust, sold in paint and hardware stores). Apply this like a poultice along the stained area, and keep wetting with additional bleach. Remove after several hours, scrub clean, then repeat if necessary.

Q: Our 20-year-old house has Sheetrock walls and this past year we noticed what appears to be nail heads showing through. If we repaint, will this cover the marks?

A: It depends on the kind of marks -- that is, are they actually raised places you can feel, or are they dark spots or stains that indicate the nail heads are rusting and rust marks are showing through. If, when you run your fingers lightly over the surface, you feel a slight bump, then it could be the nails are coming out. If so, you will have to drive them back in, and then drive additional ones in nearby. If it is only discoloration, I would spot-prime with an alcohol-thinned stain-killer or sealer, then paint in the usual manner.