Is there some type of coating I can use to prevent condensate on my HVAC ducts?
I call it “The Coke bottle” effect. When you take a coke bottle out of refrigerator, it sweats on the WARM side. 

The same thing happens when you start pumping cold air through your metal ducts in a warm atmosphere. (And vice versa for warm air.)

Paint doesn’t isolate the metal duct surface so you’ll have condensation. 

I’d look to “duct insulation” which is made specifically for ducts. Some are self-sealing and have a peel and stick adhesive so it would be easy to apply.