The previous owner of my house laid slate tile over hardwood in the entryway. A 9 square foot area has become loose and I need to re-set the tiles. Pulling everything out is not an option; I'm wondering how to make best of a bad situation.
Several products I've seen say specifically "don't apply directly to hardwood" (eg http://www.custombuildingproducts.com/TDS/TDS-133.pdf). I'm wondering if using an appropriate primer first will make success more likely, such as CBP's Multi-Surface Bonding Primer (http://www.custombuildingproducts.com/TDS/TDS-115.pdf). They describe it as "polymer-modified mortar … for tiling difficult surfaces, such as plywood, vinyl and laminates, and for hard-to-bond, nonporous tile, such as porcelain and glass."
More generally, when a tile adhesive says "don't use directly on" hardwood or another surface is this because the product will perform poorly or because there could be unintended consequences (eg chemical reaction).
Best Answer
The problem isn't the finish of the wood, it is expansion and contraction. Wood moves a lot, even more so in entries. Only 2 options: take it out and use plywood over the wood and then tile, or keep making repairs.