Q I'm trying to keep my shower tiles clean. They are very dirty and grimy. I use many household products that clean the tiles somewhat, but I want to use something that will bring them back to almost new. Any tricks? The house is more than 40 years old, so I know that's part of the problem.

Bowie

A Unfortunately, some cleaning products sold for use in showers wind up scratching the surface, making it difficult to keep them clean, says Sole Alcaino, part of the technical team at Aqua Mix (http://www.aquamix.com), which makes stone and tile cleaners. She recommends using a mild cleanser, such as the company's Concentrated Stone and Tile Cleaner or clear hand dishwashing soap. Rinse thoroughly and allow the tiles to dry. Then wipe on a penetrating sealer, such as the kind you'd use for grout, but in this case spread it over the tiles to seal the scratches. Or rub on a coat of car or furniture wax, as long as it's true wax, not silicone. Wait a bit for the wax to dry to a haze, then buff the tiles with a clean cotton cloth until there is no tacky residue. If you use a penetrating sealer, you might get easier-to-clean tiles for several years before you need to apply more, Alcaino says. She estimates wax might last for 10 to 30 days.

My husband and I stripped the polyurethane from a 10-year-old engineered wood floor in our kitchen. We removed the old, dirty finish. But there remains in spots a very thin, shiny remainder of the original finish. Before applying the new finish, do we need to sand down every trace of this, or will it blend in with the new finish? We bought Minwax High-Build Polyurethane. Is this the best finish for our floor?

Takoma Park

Once you start sanding off a floor finish, everything needs to come off or the shiny spots will wind up a different color. However, you shouldn't sand any more than necessary, because you don't want to grind through the top layer. Once bare wood is exposed everywhere, you can use any water- or oil-based polyurethane that's labeled for use on floors, including the product you mention. Similar-sounding finishes that are labeled only for woodwork or furniture contain softer resins and won't stand up, according to a customer-service representative at Minwax.

Because you have been able to sand off most of the existing finish, your flooring was probably coated with standard water- or oil-based polyurethane. Over the past 10 years, though, many flooring companies have begun using aluminum oxide finish. It's as hard or harder than most sandpaper grit. "If you try to sand it, the mineral on the sandpaper just polishes the aluminum oxide in the finish," says Dee Lenston, technical and training manager at BonaKemi, a manufacturer of floor-care and finish products.

For these floors, the trick is to first spread a product such as the company's Bona Prep, which softens the urethane in the finish, making it possible for a buffing machine to scratch up the old coating enough for a new layer of finish to stick. This approach works on any kind of floor finish. But the old finish must still be intact. If you spread a prep product on a floor with patches of bare wood, it might keep the new finish from curing properly.