Q. How should I seal marble and granite countertops? To be used in a kitchen, a surface should be waterproof, heat resistant, stain resistant, and safe for food preparation.

A. Stone countertops should indeed be sealed, but it is not a one-step process. Marble especially is very porous and requires repeated coats of a sealer. The brand I like best is Masterpiece Marble Care System (cleaner, sealer, polish) available from Ceramaseal, Box 33322, Seattle, WA 98133; 206/365-9402. It reportedly provides all of the features you require.

The homeowner should apply a thin coat of sealer to marble about three times a week for the first month, once every two weeks for the next three months, and then once about every two or three months. Granite, being denser, requires fewer coats. The sealer is absorbed into the pores of the stone, and after the first year the stone is impervious to almost everything.