Phosphoric acid must be neutralised with water. You can't just spray/paint it on and leave it. Paint it on, scrub it in (wire brush, scotch pad etc) don't let it dry, wipe it down with a damp rag, dry it, wipe it down with metho or prepsol then paint it straight away. It's good stuff, but if not applied correctly, will do more damage than good.
WURTH 0893110 is the product for you. It's great stuff.