How to tile and prime over stained concrete?

bow2021

Hi,

im about to start tiling my outdoor patio with 20mm porcelain crazy paving. I have cleaned the slab but where it has had a wet edge for most of the winter it is stained. I’ve used a pressure washer and had a go with a wire brush but still pretty stained. Will this be ok to prime and tile? 

second part to this, because if the wet edge through winter I’ve added an agi drain all the way around the patio to aid drainage and I’m going to use planiseal MR as the primer before tiling as this was recommended as a good water barrier that can be applied to damp concrete. 
thanks.

JacobZ

Hi @bow2021,

Realistically, it is unlikely to have a huge effect on the adhesion of your tile adhesive, but it won't have absolutely no effect. It would certainly be worthwhile to take one more step to see if you can fully remove it and do everything you can to ensure your tiling project is as successful as it can be. 

I'd try etching the concrete with an etching compound such as this Berger Jet Dry 1kg Active Etch. It is a citric acid-based product that will clean and texture the surface so that your primer and tile adhesive can bond as strongly as possible.

You should mix it according to the recommended ratio on the packaging, then use it to scrub the stains with a Scrub Brush.

Allow it to sit on the surface for 15 minutes. It will bubble and fizz; this is normal. Once the 15 minutes are up, wash clean with fresh water.

While it states that "rinse residue will not harm plants and grass", I would still be inclined to limit the amount going on your grass, as it looks like you take quite good care of it. Something like a temporary garden edge or a tarp should do the trick.

Let me know what you think.

Jacob