OSB

[uncle harry]

I didn't read the whole thread so not sure if this has been suggested already. I would use "Kilz Primer" which does a fantastic job at cover stains, smoke and fire damage as well as the smells associated and then paint with maybe a semi gloss so you can clean if needed or just leave it in primer.

Zinzer 123 is also a good stain blocking primer.

 

[rgray]

I use drywall mud

I also did this. My dad did his shop this way. and I did it on his suggestion. I thought his was drywall, he used more of the mud, spreading it over the OSB in a thin layer before painting. I misunderstood that and just used it on the gaps left on some of the joints.
I'm sure his walls took alot less primer and paint. His shop is 15 years old now and still looks the same as when done.
I used the kilz primer also. An airless sprayer gets it done fast.

 

[4GSR]

Good old sheet rock works for me!

 

[Bob Korves]

Studs, rafters, and tar paper showing works for me, at least until I move somewhere else. WAY too much crap in the way to start over. I have been here 25 years, so pretty well settled in, but my SO and I plan to sell both our houses and be together in a place more suitable to our current and future needs, so maybe there is still hope for a fresh start with a new shop! (and a helluva lot of work...!)