Is it possible that the stain is tannic acid from the subfloor?

(for about an hour) and according to the manufacturer, that is okay (only w/ white distilled vinegar and not an acid based descaler)Hope that helps.

I used a water-based stain instead of acid-based, and it was extremely simple.

A third use is lead acid for batteries.

You need a special license/permit thing to store chlorine and acid, and if they ever leak and mix together, you'll be in huge trouble.

Pouring a wheelbarrow load of boric acid and sugar maybe will cut them down.

If the floor is only stained I don't see why a good etching with acid wouldn't open the concrete pools again as well for the mortar to stick.If the floors have a clear coat no matter what you put on top its only going to bond as strong as the clear coat bonds to concrete so it may need to be removed.You can test clear coat adhesion by scoring an X in the floor then placing gorilla tape or similar over the X and see if anything pulls up.

Make sure the company has been in business a long time and that they do the acid etching prior to the spraying.You can have the sink done too but honestly, I think a few bucks more for a new one will give a better impression.  

Here is a secret to get rid of it, but I recommend you doing it outdoors because the fumes will be very strong- muriatic acid - dilute 1:1 with water and scrub away. 

I am presently testing this recipe promoted on YouTube and other home repair site:-Mix equal parts sugar and boric acid powder-Place in jars or small containers near infected areasI placed my test mixture into a small petri dish and so far, haven't seen them drawn to it like some have claimed.