Q.: We have a problem with a concrete stain product. It appears that the stain did not truly penetrate. We are told that the use of liquid curing compounds created a barrier to the stain even though the contractor applied a paint stripper first. These areas need to be re-done, but we would like to know what alternatives are available. We want to avoid beadblasting or sandblasting since that would expose part of the aggregate and wouldn't match the other smooth concrete areas. Any suggestions?
A.: We would guess that there still is curing compound on the surface of the concreteùespecially in the pores. If you throw a little water on the surface and don't see the concrete immediately look wet, it's a good sign that the curing material is blocking entry. It's definitely best not to put anything on top of fresh concrete when acid-etch stains will be used.
These days, contractors who do staining frequently sand the surface before applying stain. It helps to open up the canals where the calcium hydroxide (the compound that the stain interacts with) resides. We suggest that you choose an inconspicuous place as a sample area, then try sanding and re-staining. To sand the floor, contractors usually use 17-inch buffing machines with a sanding head attachment (40- to 80-grit sandpaper is most commonly used).