For the undercoat, I typically use a flat black primer (when I can find it) or flat black enamel. Over the last two years, a number of spray enamels have been used. The ones I recall are shown below.
During the winter, I typically spray in the garage and then bring the figures inside to dry in a well-ventilated storage room. Often that works to cut down the gloss finish; other times not so much.
On a trip to Home Depot this winter, I spotted the Quick Color brand. It was only $0.97 a can vs $3.50 - $4.00 a can for the products above. I brought it home and the result was a dead flat finish. Cheap and effective. Hope that result continues. The Greek hoplites are sprayed with Quick Color and the result is a flat black finish.
For the final topcoat, I spray a flat, clear sealer. I have tried Krylon products but have never had satisfactory results. I know painters who are satisfied with Krylon but for me, it never produces a dead flat finish.
The product most used is Dull cote whether it is from Testor's or Model Master. At one time, I could only get the Testor's brand but now both are equally available. Both products appear to be the same formulation. Again, temperature and humidity seem to affect the results so a quick spray in the garage and then into the storage room reduces the tendency for glossy results.
The problem with Testor's dull cote is the recent change to the can and formulation. No longer can I find the black and yellow can. The white and blue canned product still produces a flat finish but reliability issues have been frequent. Reliability issues? Yes. Every third can seems to ooze the dull cote liquid from the base of the nozzle while spraying. This liquid pools up in the lip and then liquid dull cote runs down the can and either onto the garage floor, my hand, or onto the painted figures. The can still produces a spray but I usually put the can aside once it begins oozing from the base.
What are your favorite flat black and flat clear products that are readily available?