Acid wash?

[HoneySuckle]

Why does the tile contractor recommend using an acid wash on my kitchen floor (concrete) before installing tiles? The floor has been exposed for several months since we removed the vinyl. Most other contractors said it needs some prep, but no one mentioned acid wash. Is this normal procedure?

[hj]

Partly to etch the floor so the new adhesive will stick better, and also to remove any remaining adhesive, wax, etc.

[jadnashua]

Sprinkle some water on the floor and see if any of it is absorbed. If there is a lot of old adhesive or other contaminants on the floor and water isn't absorbed, the mortar won't stick. An acid wash may not help...usually it takes mechanical work to get it off - either a razor scraper or if really bad, a scarcifier (basically a big grinder to take the top layer off). If the concrete has a really smooth surface, etching it can help. The better scarcifiers come with dust collection. They can also help take off high spots to level the floor.

If water gets absorbed and doesn't just bead up, use of a thinset mortar specified for use over cutback will suffice. This will usually be one of the better, modified (laytex addatives) thinsets.

Are there any cracks in the existing floor? They need to be handled, or they may telegraph into the tile. If any of the cracks (if there are any) are at different heights (one side either rose or fell in relation to the other), generally, the recommendation is to not tile it.

[HoneySuckle]

No cracks, which is amazing. We did scrape as much as we could off, though there are small areas that might need a little more scraping. That was HARD work

This is what he plans to use: Ultraflex II gray, and sanded grout. Sounds okay? Water did not absorb.

20 x 20 porcelain.

[jadnashua]

As I understand it, if it won't absorb water, whatever is left will act like a bond breaker for even the best thinset. Ask this question over at www.johnbridge.com. There are lots of tile experts over there. Unless I'm missing something, though, I think I'm right. Doesn't hurt to double-check.

You are using a razor scraper on a pole? Yes, it is hard work. If you can't get enough off, or if it actually penetrated the surface, you may need the scarcifier.