All of my painter friends use Sherwin Williams.

Aside from power washing and sanding thoroughly, is there any more prep work I should have my painter do?

The rock wool is non-combustable, thus if someone inadvertently lights a fire you will not run the risk of a chimney fire.In addition to caulking the outside of your windows (which is it now too cold to continue), you can apply a bead of painters latex along the seams of your window boxes (and door frames) and trim to cut down on draughts.    

We prepped for the painters by removing all the drapes, nails, outlet covers, etc.

Just throwing this out there:  I am a painter/drywall'er by trade... can do most anything within the home and have many years in the industry.  

Hey guys,I have never struggled this hard looking for a painter / painters. 

Quick Tip for painters.  

You don't have to completely clean the brush, just get the gunk off the outside.Paint sprayers are not for virgin painters.

Then you have your general tools like a vice grip, hammer, channel locks, flashlight, tape measure, composite shims, headlamp, box cutter with extra blades, painter's tool 5-1, wire stripper, allen wrenches, voltage detector, outlet tester, lineman's pliers, good Milwaukee demo screwdrivers, Milwaukee 11-1 with Phillips, flat heads, and 3-4 common size nuts(this is great for appliances).

NOTE:  with the oil base primer, make sure whoever is applying has sufficient ventilation in place--I've seen plenty of painters get 'loopy' from the chemicals without proper air change ventilation.