I just had a roommate damage one of my countertops with phosphoric acid. I've been told there is no fix except replacement, which is not a good option as I cannot match the pattern nor the wall tile that it's bonded to if I replace it. The damage is a light spot that almost disappears when wet or use lemon oil but that is only temporary.
I am trying a clear epoxy on a scrap piece that I purposely damaged with positive results so far. One problem that I'm having is that the epoxy alone isn't enough to completely get rid of the light spot. The other is that the coat is too glossy. Once it has set (about two days) with a very thin coat, the epoxy seems extremely tough so there is hope.
Today I am trying again and I think I'll be more successful. First, I used oil based paint of a darker color carefully applying it thinning it out as I go with paint thinner until it matched. Acetone will take it right off if I made mistakes. But the paint touch up isn't enough. Next I let it completely dry then mixed a very small amount of epoxy. The trick is to go very thin.
I'm not finished, but the gloss can been toned down by lightly brushing the surface as it sets. I'm doing it every hour, at first with my finger then using a cloth. I may have to use a dry scouring pad. The soft set is complete after 8 hours. After three days it completely sets and it's permanent although a second coat may help if the first does not look right.
Anyway, it's worth trying. The other options are to live with the stain or completely replace the countertop or floor. Good luck finding anything that will match. Manufacturers change their designs often making a partial fix difficult unless extra material was saved.
I am hopeful that this will work. My cost is my time and about $15. To replace it would be $3000. It's worth a try but use something very tough.