What do you mean by "flakey"?
Anyways, allways go with grey or white primer underneath black paint. And best temperature for painting is some 20-25°C, as this will make sure that the paint dries faster but not too fast. I've painted at 15°C today aswell, but the result was orange-peel as expected due to the paint taking a bit longer to dry.
Additionally. Allways sand every layer with 4-600 grit before applying the next layer. Only the final clearcoat-layer will not be sanded ofc.
Here's how I paint: fillered and prepped with 400 grit -> primer -> wait 30 minutes -> 400 grit -> primer -> let dry for 24 hours -> 400 grit -> black -> wait 30 minutes -> 400 grit -> black -> let dry for 24 hours -> 600 grit -> clearcoat -> wait for 30 minutes -> 600 grit -> final clearcoat.
I also make sure that the clearcoat and the black are of the same kind of paint, e.g. both have to have the same basis like water or acryl.