I'm going to be media blasting a bunch of bolts and screws. Is there a dip or spray coating that can be applied to retard rusting?

 

You can replate them.

Eastwood Tin Zinc Electroplating System

 

Eastwood also makes a (gloss and satin) clearcoat for bare metal that works well and a black phosphate kit that works well. You still need to clearcoat the black phosphate. I don't really care for the dull look of blasted hardware so other options...have them zinc plated in bulk. I haven't checked into the cost, but it should be reasonable...or, by the time you blast, coat, dry, handle...etc...might be easier to just buy new hardware! It might not be as expensive as you think. You can find the part numbers in the Auveco catalog...they have a really cool download and you'll be surprised what you find in browsing. You can google or ebay the part numbers and buy the hardware in bags of 25/50/100 for nut much $$...check it out. Time is money.