Outgassing problem on lamp reflector

On a parabolic reflector used with an HID lamp, discoloration is taking place on the coated surface. The base material of the reflector is aluminum 6061-T6 per astmb209 or ASTM B221. There is a 10-16 microinch roughness height called out on the parabolic surface and the coating is specified as follows:

MIRROR, ALUMINIZED COATING IN ACCORDANCE WITH
MIL-M-13508 WITH A MAGNESIUM FLUORIDE PROTECTIVE
FILM TO WITHSTAND TEMPERATURE OF 500 °F

Apparently, due to the high temperature environment created by the HID lamp, outgassing of the coating material is occurring, primarily the fluoride. I would greatly appreciate any information and advice regarding this matter as well as any suggestions of possible alternative processes.

Thank you in advance,

A spec is a spec, and if the customer wants MgF, then...

However, it seems to me a film of Al2O3, ZrO2 or even SiO2 might be adequate protection for an aluminum visible light reflector (HfO2 if you really need thermal protection); the caveat here is without more information on mechanical stress, thermal cycling, environment and other parameters the part may be subjected to, it is hard to give a definitive answer.

MgF2 is used in optics frequently as a polarizing material and because it is fluorescent under electric lights. There maybe reasons for the MgF2 spec other than protecting the reflective surface.