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.
...and allways remember, that the world is an orange!
Only when the last tree has been cut down; Only when the last river has been poisoned; Only when the last fish has been caught; Only then will you find that money cannot be eaten.