Prior to the ultraguard coat, I patched with mortar mixed with liquid acrylic (instead of water) - obtained from Home Depot.

2 years later now, and I had two 6-8" bubbles form, then break in the ultraguard. The plaster subsurface was still stuck to it, so a failure of the plaster - not the ultraguard. I used a tintable epoxy product called All-Fix to patch these holes: http://picasaweb.google.com/paul.mck...ardWithAllFix#

So far, a very good solution that nearly matched the color and is as close to invisible as you could hope for. This stuff is like putty and cures underwater.