I bumped into this little item about using friction welding, to fill in those holes that you sometimes get in your 3D prints.
The premise is based on the fact that plastic has a relatively low melting temperature, a temperature easily obtainable with friction. If you can rub the plastic against an object fast enough, the friction will cause enough heat for that plastic to melt. This melted plastic (3D printer filament in this case) can then be used to fill in holes, gaps, and other blemishes on a 3D printed object.
Welding two plastics together seems fairly straight forward. I think using this process would be a bit tricker to fill small holes, since the object has the same melting point as the material you are filling with.
On the artistic side of things this technique offers the potential to add additional color and designs to complete your printed object. Makes me wonder if something like a wood burning tool, at the correct temperature, couldn’t be used to a similar end.
Source [with video]: http://3dprint.com/16023/friction-filling-3d-prints/