I use fingers
naa believe it or not i use white spirits, i brush it on, leave it for a min then work it about with a brush, then ill go over the area lightly/quickly with my little butane burner. It has to be left for a few mins after as the clay is then very soft, i could speed up that time with a can of compressed air to cool it down, but i just work on somewhere else normally hehe
for finer details i use talc on a small brush to act as sandpaper, as the burner and white spirit are too harsh.