If the goal of the filters on the wall fans is to remove the visible dust (3+ micron) that settles around the shop, then a MERV 8 would probably do the job without restricting air flow too much, at least until the filters get clogged. Merv 11 is the minimum I would use if the goal is to reduce the amount of finer dust you breath. MERV 13 is sort of the sweet spot where you get above 90% of 1-3 micron particles at a relatively affordable cost. Remember what I said on one of the other threads about how fan blades without cowlings actually suck in air from the front of the fan so if you are pulling the air through the filter rather than blowing air into it, it may be that not all of the air coming out of the fan will be filtered. A cheap (Amazon) handheld anemometer is pretty handy for evaluating airflow for projects like this. You can actually see the air flow reverse direction around the edges of the fan, indicating that it is pulling air in around the edges.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
