More than likely you're fighting with the pressure sensitive adhesive that's down in the cracks. It's possible you'll never get it all out, but there are a couple of things you can try. First, make sure you've removed as much of the pressure sensitive from the slab as possible. Dry scraping is o.k. here as long as you're sure there's no danger of asbestos in the glue or from an old installation. You'll need to pay particular attention to the cracks themselves. Take your time and scrape as much of the pressure sensitive out of the cracks as you can manage. It's tedious work, but with a couple of guys, you can get it done in a reasonable amount of time. Finally, prime the floor around the cracks with a good latex primer. Many of the floor patch manufacturers make these products for use with their patch in certain situations. Make sure you get good coverage with the primer down inside the cracks, but don't allow it pool. If it dries too thick, you'll be back where you started.

This is all going to take quite a bit of extra time, so make sure your customer knows about it ahead of time. Let them know that this will probably not completely eliminate the cracks, but that it is the best option available and you believe it's worth the extra effort. 

 

Hope this helps,

 

Scott