Mark - Here's the method that I use to make sandbags.
1) Using A-B Putty, I combine the two parts and then roll it out into a long "snake".
2) Flatten the whole thing out a bit
3) cut out a bunch of sandbag lengths
4) press a seam into the edges with a toothpick
5) apply to the model, molding it to the contures of the vehicle at the same time
6) press the exterior with some course material to leave an impression
7) allow to dry, then paint and weather.
Happy Modelling and God Bless
Robert