These are some of the brands of fillers and putty I have bought. Didn't really like the powder Water Putty because I had to mix it with water and found it messy. May try it again.
Here is another raccoon, this time an 19 lb one that stood up on its own, but fell over if easily pushed in the front. Rather than take a chance of this heavy dude falling over, I decided to build the back of him up so it would sit up straighter, instead of leaning back so much.
I decided to use the 3M Wood Putty on this one, no particular reason. It comes out like thick paste.
I used my fingers to smooth it out. It's thick, so hard to make it really smooth. I helps to have a damp finger to help smooth it. Don't put it on too heavy or it will take too long to dry and may crack. I put two thin coats on this raccoon to build it up to the level I needed for it to stand up better.
After it's had time to dry it should be rock hard. You can use sandpaper to smooth the putty out. Try to make the edges smooth so that it blends into the rock as one.
After I sanded the putty I painted it to match the rest of the rock. If you do a good sanding job you won't even be able to tell that you used putty or filler. I noticed after I painted mine that there is a spot that could have been sanded more, but not too noticeable.