Decorative problem clinic. (Decorative Concrete).

Coloring concrete slabs

Q. I am looking to finish my store's concrete floor but it has a very smooth slab finish. Someone told me that spreading fertilizer and grinding it into the floor would add a nice color and make it porous for the stain. Have you ever heard of this?

A. We wish coloring concrete were so easy! What you've probably heard is that one can place acid-etch stains on concrete and then sprinkling plant fertilizer on the stain to produce additional colors. A fertilizer (such as Miracle Grow) has copper salts that will impart green and blue highlights to the stain. Others, rich in iron salts, will produce brownish affects. Some contractors have even placed the fertilizers on the slab and sprayed weak muratic acid solutions over them to provide some coloration.

Be sure there aren't sealers on the slab that would block the stains from penetrating.

Sealing exposed aggregate

Q. I am having trouble sealing my exposed aggregate driveway. I'm told to do this once a year but the sealer won't hold up that long. The first time I used a water-based sealer that turned a milky color after 3 months. Next, I tried a brown tint acrylic sealer that did the same thing after 6 months. Last winter was hard and I noticed a few rocks popping up. Across the street, a neighbor washed his driveway one day and sealed it the next. I had been pressure washing in the morning and sealing in the late afternoon. This time, I tried his way. With a sudden, unexpected rain the next day, I now have a few milky spots and tire tracks. Can I apply another coat on top? Or do I have to pressure wash and start over? Am I doing or not doing something to cause the milky white spots?

A. This is not a simple problem, and water-based sealers complicate the issue further because moisture and temperature conditions are more critical than with solvent-based systems. Often, whitish areas on sealers indicate that the sealer has lost its bond to the concrete and light reflecting through that small air space causes the white appearance. If that is the case, all the white marks must be completely removed prior to resealing. Another possibility with water-based products, is that if the coalescing agent in the sealer did not evaporate in the proper sequence in relation to the water in the sealer, the acrylic molecules will not join together in the proper manner and that will create the white appearance. In this case too, the white material must be completely removed because it will reflect through a new coat of sealer.

You might start with a pressure washer. You can go as high as 3000 psi and bear down on the surface to remove anything that is not bonded well. Pay close attention to the concrete to be sure you aren't removing any part of the surface. Then a stripping agent should be used to remove any white colored areas that remain. A contractor recently told me that he used a product called "Citrus Strip" by Dayton Superior that worked well for him (after trying two similar products that didn't). He used a buffing machine with a strip pad to work the stripper into the sealer. You can also use solvent strippers, but if you do, find one that has a surfactant included, which will allow the dissolved sealer to mix with water for easy clean-up.

In general, we don't recommend water-based sealers outdoors, preferring to use solvent-based acrylics--there are fewer problems with them and if there are problems, they are easier to repair. The best seem to be 100% acrylic sealers--ones that do not have other polymer additives. A major concern is the "breathability" of a sealer in an outdoor application. Acrylic resins transmit higher amounts of water vapor than other polymers. Your local construction supply store should be able to help you find such a sealer.