Q: I have a thick slab of natural marble set into a wood frame that serves as the top for my coffee table. This piece of marble has cracked in half. Can it be repaired?
A: You can join marble successfully with an epoxy adhesive that comes in two separate tubes that must be mixed before use. It comes in white, clear (actually amber) and a gray or metallic color. I cannot say which will be least noticeable, but you will see the adhesive line when the parts are joined. The epoxy will form a strong joint, but you should support the marble under the crack if you are using it for a table top. The mended slab should rest on a sheet of plywood or similar backing to support the weight along the joint.
Q: I have a light gray marble mantle over my fireplace. The marble has a yellowish stain on it. I have used a soap-impregnated steel-wool pad to try to get rid of the stain, but with no effect. Can you tell me how to get rid of it without damaging the marble or changing its color.
A: You have probably already damaged the marble, or at least scratched it, with the steel-wool pad. Stains in marble have to be treated with a poultice -- a paste made by mixing powdered white chalk with a suitable solvent or bleach. You also have to know what caused the stain to know what kind of solvent or bleach to use. Since you do not have this information, your best bet is to buy a marble-cleaning kit sold in department stores and home centers, or by marble dealers. These have an assortment of solvents and cleaning compounds, one of which may work better than others. Polishing will then be required to restore the gloss and remove the scratches, but this should be done with a very fine abrasive such as a marble-polishing powder or powdered rottenstone.
Q: We had a leak that allowed rain to soak into and stain the plaster wall on the inside of our house. Since then, the plaster regularly chalks off or foams and I cannot get paint, or even topping cement, to stick over the plaster. Can you tell me what I can do to correct this?
A: When plaster has been saturated with water it is no longer a sound structural material and no amount of patching or covering will make it suitable surface for painting. The only cure is drastic surgery: Cut out and remove all the defective plaster. Use a wide chisel or wrecking bar and a hammer to chip away the old damaged plaster, going all the way back to the lath or gypsum board backing if necessary.
Check the condition of the lath behind the plaster. If it is rusted or rotted, it will have to be removed and replaced with new material. In many cases of this kind, instead of replastering the damaged areas, the owner simply patches with pieces of plasterboard (gypsum board). These are nailed up over the studs, then patching plaster is used to fill in any gaps around the edges where the plasterboard meets the original plaster. Final smoothing over is done with spackling compound or joint cement.