I use a brand of paint called Halfords, from the Halfords store here in the UK. Its fantastic paint.
I would use 3 full 300ml cans on one helmet, building the paint up bit by bit so the helmet has a really nice thick coat. Let it dry for at least a week.
Use the finest wet and dry paper you can find. Wet sand it and keep drying it with a towel to check for little glossy dots. These gloss dots are the low spots of the paints texture thats not yet been touched by the sand paper.
When the paint has all been sanded smooth, leave it for another day or two, as the paint can be soft again now its had the top layers sanded away.
Any brand of T-cut should do the job. If you do it right then you may not even need to polish it, but if i do use polish, i use autoglym super resin.
The following helmets were painted inside and out using the same method and its impossible to tell they have been painted even with the closest inspection.
Keith.