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yep, painted surfaces radiate heat very much more effectively than bare metal, so they send much of their thermal energy to (very cold) outer space and get cooler than the air in the process - hence dew.
I found that the CF tube on my Newtonian was radiatively cooling below ambient and dewing up, so coated it with aluminised tape and the problem went away. Maybe do the same with your secondary - even a bit of alfoil taped over the back of the holder should help. The advantage of passive techniques is that you are less likely to get air turbulence and resolution loss due to thermal gradients - which will happen if you use heaters.
rough or dark material of any kind, most paints, even glass (eg lenses and corrector plates) are all pretty good thermal radiators and will cool below ambient and dew up if they are pointing at the sky. Clean bare metal is the best material for resisting dew formation.
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