I may be teaching you to suck eggs here but...
My tip for getting a smooth finish by spray can.
Degrease the model, tape up any areas to be masked (you dont have any on the stairs) and arrange the method of holding the model for spraying. I use old wire bent into a hook so that I can hang it.
Then hang the model somewhere warm. The airing cupboard is ideal.
Then put the kettle on.
While the kettle is boiling up hold the can of paint upside down and shake it for two minutes.
Then pour the hot water in a suitable container and put the spray can, the correct way up, in it so that the contents can warm up.
Then go and prepare the work area where you are going to spray.
When the model and paint are both warm, give the can another good shake and begin spraying.
Start by pointing the spray can off to one side of the model and about 12 inches away. Then press the nozzle (this stops the blotchy paint finish) then quickly pass the spray over the model in horizontal strokes working downwards. Make sure that that you start to spray off the model, across it and spray air again before turning back to do the next stroke. i.e don't hold the spray stationary on the model.
Then wait a little while (put the can of paint in your pocket to keep it warm) and repeat this time painting vertically.
then do the same again horizontally etc etc.
Each coat should be very fine. At first it should very patchy. but at the end you should have a nice even coat.
Now here is the clever bit.
When the smell has gone away...put the painted model back into the airing cupboard to harden.
I have been led to believe that the warm atmosphere encourages the molecules in the still soft paint to even out. So that you get a smooother and harder surface.
Of course it could be a load of old tosh, but it works for me.