Well I 4 one have been using PPG's products for 30 years PROFESSIONALLY , and have been to about every PPG refinish school and am Platinum certified
ALL LEVELS................
You might want to go to one mgbgt as PPG STILL reccomends using DP/DPLF sprayed over BARE metal then Apply the bondo over that !
Then you can spray K36 or NCP 271 ( which I have used since it came out 5 years ago.)

AND adding fiberglass resin to your bondo is NOT reccomended but the HONEY specifically designed for bondo is for a smoother spread out.
I have my body men cut their bondo down with 80 grit then apply a thin layer of glazing compound over the bondo repair then cut it with 180 grit.
This will insure with having to use many layers of primer for filling and less shrinkage !

SPRAYTECH