Craze cracking, dusting, blistering, scaling, random cracking, and other surface defects sometimes are caused by using the wrong finishing tool, or the right tool at the wrong time. Most finishing tools do one of two things: They either keep the surface open, allowing mix water to escape, or they seal and densify the surface, preventing mix water from escaping. Floats usually keep the surface open. Trowels close or seal the surface. For good results, avoid the finishing mistakes described in this article. JITTERBUGGING HIGH-SLUMP CONCRETE Grate tampers, or jitterbugs, are used to push coarse aggregate particles below the concrete surface right after strikeoff. You should use jitterbugs only on low- or medium-slump normalweight concrete or lightweight concretes with slumps less than 4 inches. USING A FRESNO INCORRECTLY A fresno is a steel trowel attached to a bull-float handle. Using a fresno while the concrete is still bleeding seals the surface. This stops further bleeding but traps bleedwater beneath the surface and can cause blistering, surface delamination, or deicer scaling. USING THE WRONG FLOAT ON AIR-ENTRAINED CONCRETE Wood floats work best on non-air-entrained concrete. Instead of wood, use magnesium or aluminum floats on air-entrained and lightweight concretes. TROWELING AIR-ENTRAINED CONCRETE TOO SOON Use floating as the final finish for air-entrained concrete whenever possible. If the owner requests a steel-trowel finish for air-entrained concrete, start troweling only after bleeding has stopped.