You just gotta love plaster. That rock-hard substance, which was applied to the walls and ceilings of nearly every house in this country until the 1950s, gives us surfaces that are seamless, mold resistant, fire resistant, and noise deadening. But what to do when plaster cracks, buckles, and pops loose? It's a perplexing question for many of our readers, including Tim Thorp, whose house in Providence, Rhode Island, is filled with badly blemished plaster. "How do I patch 100 years of gouges, cracks, and screw holes so the walls look flat and clean when painted?" he asks us in an e-mail. Here, Tom shows how sticking old walls back together can make them look as good as new.