I actually rarely use putty when rebasing Reaper miniatures on spare GW 25 mm bases. Typically, I take a wire clipper and cut off as much of the Reaper base as I can without damaging the figure, glue it on the GW base, and prime. Then I coat the top with wood glue, strategically place some fish tank rocks in the nooks of the now removed Reaper base, and sand on the rest. The advantage is that I don\'t usually have the sand/rocks going above the feet of the miniature. (I have a whole friggin\' bag of fish tank rocks; it\'s gonna last forever...) After painting the whole miniature, I then cover any part of the base I don\'t like with Static Grass.
A good example:
http://www.coolminiornot.com/index.php?id=8114
You can see on the right side of the front view where the reaper base ends. On the left side, there\'s a pebble to disguise the level change, and on the back, I mask most of the line with another pebble and a ton of static grass.
The benefit of doing it this way? Bases only take about an hour for relatively good results; especially when I drybrush six or seven figures at a time.