I regrouted a shower using an old self-mix cement-based grout. It dried fairly quickly, but when I went to seal it, it dissolved in the sealer. I figured it hadn't cured yet, so I waited a month and then went back to seal it again, this time with a different sealer and got the same results. I run my finger over some of the groutand it comes off like chalk dust on my hand. Is there something I can do other than regrouting the whole thing?