Sometimes what I like to do on a rough frame is to hit it lightly with a sander. Just enough to take the fuzz off the beams but not enough to remove saw marks from milling. I retains the rough cut character and also lowers the chance of getting a splinter from running your hand over a timber. Especially nice on something coarse grained like fir. If your timbers have weathered a bit it can actually increase the visibility of rough cut character. Lighter wood is exposed on the surface, but the wood in the nooks and crannies stays darker. It looks pretty sharp, and takes a lot less time than a fully smooth surface. Like Don said, if the timbers have some encrusted dirt soap and water would take that off first.