To those of us who have seen hundreds of awful sandblasting mistakes, it seems incredible that there should still be any need to warn against it. Yet people still ask whether sandblasting is okay. People have been told by contractors that sandblasting ''is the only method that will work.'' Others have heard that sandblasting causes problems, but don`t know the alternatives. In abrasive blasting, sand or another abrasive is shot against the building in a high-pressure jet of air or water. This quickly removes the paint, but as the abrasive doesn`t know where the paint film stops and the masonry starts, it always removes some of the building, too. It is all too easy to lose control; in fact, with high pressures, it is impossible to keep control. Consider the variables:
-- Type, condition, and hardness of masonry;
-- Density, hardness, size, and shape of the abrasive;
-- The pressure used;
-- The constancy of the pressure used;
-- The distance from nozzle to surface;
-- The skill of the operator;
-- Visibility of the surface;
-- The angle of ''hit.''
All of these factors significantly change the effect of the blasting on the surface. And there`s no way sandblasters can control all of them. The machinery is heavy, the scaffold has to be moved; all workers become fatigued. When the dust clears, the masonry will be changed. Agregate is exposed as the softer clay is blasted away. Depending on which material is harder, the relationship between the masonry units and the mortar will have changed, altering the character of the surface. The ''skin'' is removed, often exposing a more permeable core.
Blasting increases the surface area of the wall, and so it will now get wetter than before, and it will become dirtier more quickly, With no crystalline planes to refract light, the surface reflectively changes, sometimes resulting in an apparent change of color.
It can`t be fixed. Clear sealers often make the problem worse. Moisture will get into porous masonry somehow; it always does. But sealed masonry won`t be able to let it back out. Instead, the water will build up hydrostatic pressure behind the coating, and when it pops out, it will take some masonry with it. Also, clear sealers run from not effective at all to effective for only three to five years (you`re back to a maintenance schedule worse than painting), and applicator error can leave streaks or variations in surface gloss across the facade.
Need more reasons not to sandblast? It`s strictly prohibited by the Security of the Interior`s Guidelines for Rehabilitation--a condemnation from people who know, and you won`t get your investment tax credit. Because of the environmental hazard of flying abrasive, it`s now banned by many municipalities.
Some contractors don`t call it sandblasting. Sand may not be the abrasive used. The list of alternative grits includes: ground slag, volcanic ash, almond and walnut shells, rice husks, corncobs, ground coconut shells, glass beads, crushed eggshells, microballons, powdered limestone and chopped plastic. It doesn`t matter; any grit blasted at high enough pressure to remove paint will harm the surface underneath.
''Waterblasting'' is popular; this is when the sand is carried by water instead of air. It has the advantage of letting the operator see the surface, as opposed to being surrounded by a cloud of dust, but it doesn`t soften the blow. We`ve even heard the word ''featherblasting,'' and silly things like,
''don`t worry, I put talcum powder in the sand.''