Many times, the tile installer is placed in an awkward position by the general contractor, builder, homeowner or whomever is in charge. Many of these people believe that they know more than the tile professional as to what is needed to properly complete the job, including the substrate prep. Unfortunately, many times they are wrong, especially when it comes to a “harmless” surface contaminant such as paint.
The tile industry is driven by the American National Standards Institute (ANSI) requirements as shown here under A108.01.
2.5.1.2.2 Sound, crack-free concrete and masonry walls which are smooth, dusty, coated with form-release compound, painted, effloresced, or have loose surface material may be prepared for direct application of mortar and tile (no membrane or metal lath) by mechanically scarifying to expose uncontaminated backing surface.
2.6.2.2 (abbreviated) Other suitable substrates to receive ceramic floor tile are structural concrete of limited area and slab-on-grade construction where no bending stresses occur; also, if expressly approved by the mortar manufacturer, surface such as portland cement terrazzo and existing ceramic tile. Surfaces must be clean, free of wax, curing compound, and other coatings.
2.6.2.4 Plywood: Surfaces must be structurally sound, dry, and free of contaminants such as sealers, cleaning compound, coatings, oil dust, dirt, etc.
2.6.4.2 Concrete slabs, existing ceramic tile, and terrazzo (using an A118.3 or A118.8 epoxy mortar). Floor surface shall be dry, structurally sound, and free of wax, curing compound, or other coatings. Sound, crack-free concrete which is smooth, dusty, coated with form-release compound, painted, effloresced, or has loose surface material may be prepared for direct application of tile by mechanically scarifying to completely expose uncontaminated backing surface.
These ANSI requirements call for the surface to be clean and free of coatings which includes paint. They further call for the removal of these products by mechanical scarification which actually removes approximately 1/8” of the top surface, thereby eliminating the contaminant.
Unfortunately, the folks listed above don’t understand the physics of the process, which requires a good mechanical bond to the substrate. They don’t realize that the paint or other coating is a bond breaker, rendering many dry-set mortars useless as seen in the image. However, it is true that some of these mortars will bond tenaciously to the paint and yield good results. The problem in this instance is not the bond of the tile and the modified dry-set mortar to the paint, but rather the bond of the paint to the substrate, be it concrete or plywood.
Today’s mortars, especially the A118.15 improved modified dry-set cement mortars, have great bonding characteristics that, most likely, will have a higher bond strength than that of the paint which is now compromised. The affected area is now loose, exhibiting a hollow sound which is the front runner to crumbling grout joints, broken tile and an expensive callback, giving the tile industry yet another black eye.
As a high-quality tile professional, stand your ground, say “no” and do not allow anyone to coax you into believing the paint is okay “this time” and just get the job done. It may be better to say “thank you” and walk away.
