This is my method on how to keep a painted finish on headers.

Basically the painted finish (using hi-temp paint) has to be baked and most of the time the
oven is too small.

Step 1 - Remove headers from vehicle, strip finish off headers and clean thoroughly. Remove
any grease or oil.

Step 2 - Paint headers with a name brand high temperature paint such as VHT.

Step 3 - Reinstall headers, be careful as not to get any grease, oil or dirt on painted surfaces.

Step 4 - Fire up the motor, run it for about 1 to 2 minutes.

Step 5 - Let headers cool.

Step 6 - Repeat steps 4 and 5 four to six times. The more the better.

The key points are;
Headers must be absolutely clean before painting.
Painted surfaces must be clean before the baking process.
Do not let the headers get too hot while baking.

To maintain your newly painted headers keep them clean, use soap and water (while cool).
The paint job can be touched up but the new paint has to be also baked.