thas another way dave...I tried that on fiddly assets in PS and it was a pain in the butt...IF the faces are on 'joins' with different textures I found my method much quicker and simpler...again this was on a building so it worked for that kind of geo.

If you go daves way make sure you have a good size res....and if you make a mistake with the pen tool (dave?) I believe holding down CTRL you can move the anchor points etc...as well you can add or delete anchor points with ALT and SHIFT..I cant remember...dave may know these shortcuts?

cheers bullet