Personally, I stay away from liquid, paint or roll on membranes. Now, if I did this on a regular basis, that may be a different story. I think it is hard for the DIY'er to get the proper thickness without getting pinholes or thin areas; excessive thickness can be an issue, too. What I'm talking about is something like Kerdi http://www.schluter.com/5288.aspx this goes on more like wallpaper than paint (it is a sheet membrane). Noble, and others may similar things. To me, this is more reliable. FWIW, Schluter gives a 10-year warranty if you use their stuff and install it properly.