I have not done any staining of our tiles, but I have used a couple of products which put a new seal on the tiles. These were water-based products that I applied by just cleaning the tiles really well and then brushing on a layer and letting it dry. I have used several products and would not recommend a particular one because I'm not sure how the ones I have used would react with the seal you already have. My suggestion is to buy a small amount and try it on a small area. That is exactly what I have done. Let it dry and set for a week and then see if you like it before doing the whole floor.