You might try sanding with a rotary (not orbital) sander/grinder with something like 24 grit, possibly followed by a rotary wire brush that will remove some of the soft grain. I have done this successfully for ceiling beams.

I have made many rough sawn 2X6's using finished lumber. I found that running a jigsaw or even better, a sawzall across the board face works well. Lay the saw blade flat on the lumber face and move the blade slowly with the top of the balde as your leading edge. The more often you repeat it, the rougher the board surface will become. GOOD LUCK!