Primed Ductwork
There sure are a lot of pieces! I use a water-thinnable epoxy primer for the pieces that are not subjected to very high temperatures. It's nontoxic, so I don't have to use a respirator, and that makes it very easy to use. It is especially rugged and withstands fairly high temperatures (250F or so). It will take a couple of days to dry to full hardness; then I'll give it a light sanding and apply the color coat.