I have personally converted glossy to matte with great results, using Krylon Matte Finish Spray #1311 from Walmart (about $3.75 a can).
I've done over 30 metal figures and the can still seems full.
I have also conveted matte to glossy, with great result, using Krylon Clear Glaze Spray #0500, also available at Walmart (same price).
Both of these products work very well and are an exteremly cheap conversion method.
The can states it won't crack,can also be used on plastic and should be applied in a low humidity 50 to 85degree inviroment.
Try it on a cheapie, it worked well for me,
Michael
Don't touch until dried or you will leave a fingerprint.