Repainting decoys is time consuming and a lot of work, so the longer you can make them last the better off you are. If you are interested in using a good clear coat over the decoys you repaint, I found a really good one. I have had good luck with either Krylon or Valspar clear flat, but I found something that works far better.
It's a 2 part urethane automotive clear coat I got from a company called Transtar. Now, you have to mix it so it's more work than just taking the lid off a can and pushing a button, but there's no comparison in the results...this is head and shoulders above either Krylon or Valspar. You will need a well ventilated space in which to apply it, and you MUST wear a good mask, but it's very easy. After mixing you get an hour of pot time before it starts to harden. You can use an HPLV spray gun from Harbor Freight and it works just fine, so no need for super expensive equipment. I have not tried it through an airbrush. My rough estimate is you can do around 1000 decoys from a one gallon kit for a cost of about $.15/decoy. It works best if you "bake" the decoy after application, 140 degrees for 45 minutes, BUT it works just fine air dried as well. If you air dry it the sheen is about the same as if you used a flat clear coat from a rattle can. If you use the heat method it is dead flat. I would think your attic or a metal box in the summer sun would be about the same as baking it.
It's a little more work, but in my opinion it's well worth the extra effort. The flattening agent for the clear coat was added at the factory, so no need for mixing that in.