Using 600 to 800 grit wet and dry abrasive paper, rub down the entire silver reflector.
This keys the area allowing the paint to stick properly.
When you have rubbed the reflector down, clean all the dust off and dry it off.
When you spray the reflector, ensure that the temperture isn't too low.
Spraying in a low temperature makes the paint finish matt. I heated the reflectors
with an electric heater before spraying. I left the heater going to increase the
temperature in the area. Do not aim the heater at the part as you spray it as the paint
can dry too quickly and affect the finish and bond to the reflector.
Ideally you should spray parts on a hot day and leave the part to dry naturally in the sun.
Don't attempt to paint the entire thing to the colour you want in 1 go.
Spray light coats and leave for 10 minutes before applying the next coat.
Allow the reflector to dry for a few hours before refitting into the headlight.
The hardest part of this is fitting the light back together without it being full of dust.
Use a tack cloth to really give everything a good clean before reassembly.
On refitting the lights you may have poorly aimed beams. Get them set at a local garage.
You could always try and sort them out yourself by looking at the info here.
Many thanks to Tony for the extra info on spraying and cleaning the light units.