Grip paste.
I have used many different types of product for modifying grips.
Personally I find Milliput is the easiest to use, either the standard or superfine, I prefer it to other types of epoxy resin fillers because you can use a damp cloth or small brush to blend the edges. I don’t know of another product that allows you to use water in this way. It is also much firmer when mixed, which allows you to shape it by gripping the pistol grip with a wetted hand. It takes a bit longer to set, but this can be speeded up by placing the grip on a radiator or in a warm oven – warm not hot!
The section on Grip modification on this website is quite helpful:
Pistol-shooting.com
By Allan Lozsan
There is also some very good advice for keen pistol shooters in a series of notes by the late Don Nygord.
http://www.australiancynic.com/NYGORD.htm
"This Is My Truth Tell Me Yours"