Martin,
Spraying in small areas is quite an art form in its own right. I recently repaired an area on my bumper by the following.
Get some paint mixed at either Halfords or a paint suppliers. I have tried the off the shelf paint or even the dealers in the past and have never had a success with them. To fill the small areas i used a high build filler spray that has a etching property in it. Again i get mine from a paint supplier in a spray can.
Rub down the area to be painted with some 400 grade wet and dry paper. Stop when you get to the undercoat paint. This will usually be another colour. Sometimes a grey or beige. Next rub this down with a finer paper, i used an 800 grade.
Mask up the area to be painted so as to not to cover things like lights and trim that would take a long time to clean up.
Spray the area in small doses. Wait until each coat dries. The filler primer takes nearly 24 hours to go off if you paint over it before hand it will craze and look crap.
Finish off with, if the car paint is metallic, a 2 pack clear coat spray. You can get this now in a can and it works a treat.
Finally rub down with either some more fine grade wet and dry or a cutting paste not unlike 'T' cut.
With time and patience you can get a good finish, not as good as a professional but acceptable for most people.
Good luck.

8-)
DaveL