I would belt sand the beam first, to get rid of the really rough pats, then go to a finer sanding method to smooth it out. Don't know about priming though.

Thanks for the info. I forgot to mention that the beam has a dk stain on it. House was built in 1980. This is a main support beam. Should I clean it with TSP before priming? Which primer? Would prefer not to use an oil based one, due to odor.

Depending on the wood type, youre probably going to have to use a oil primer to seal knots from bleeding through, if there are any in the wood. Kilz ( which also comes in a water base) is good. There are other primers also that are made for priming over stain and hard to cover areas. Read the label on some of the most popular ones , such as Zinnzer, Binz, or Kilz. TSP is mostly for painted walls and the likes, not really for wood, so i would just get it clean with a rag if its that dusty. The primer will do the rest, (unless its oily or something) Also they have great additive products at the local paint store that you add to primer or paint to eliminate the odor. They do a pretty good job. Just add it to the paint, thats it.

Had great luck with Zinzer water-based primer on a recent paint project on knotty pine paneling. However, I do notice some sap on this beam, so maybe oil-based is the way to go. Guess that means I have to use an oil-based paint as well?

You will want a oil based primer and stain blocker at that otherwise it will bleed through.

You can use a latex finish paint over the oil based primer an dit will do fine.

Thanks-you have all been very helpful. One last questions-what grade or grit of sandpaper should I use?

It depends on how smooth you want get the wood but i would think you would want to start with 80 grit and work your way down to 120 grit sand paper. You will find this at your local paint store along with the paint you will need.

The rough cut beam are you trying to smooth it or just etch it for the primer?
If you think you need to sand it 1st before priming use a fine grit to keep the rough look board or use a coarse grit to smooth the board down some.
if you just sanding for painting purposes then use a 120 grit the prime.