Pre treatments that have ill effects on galvanneal coating prior to powder coat

Dear Tami:
I have only worked for a few sheet galvanizers so please consider what I say as somewhat remote to your problem. The worst case of galvanizing pealing that I have experienced was on re-phosphorized aluminum semi-killed steel. While working for one USA sheet galvanizer whose product was used for deep drawing in automobile manufacture, one of the biggest problems was flux control. I do not know the source of your A40. If it was from a hydrogen/inert gas reduction of surface iron oxide then there is no flux.
With galvanneal the product is heated long enough (after galvanizing) that the entire coating is a zinc-iron alloy and this alloy I think is primarily 6% iron/94% zinc. This coating is bonded in a perpendicular direction from the steel surface and should not peal.
In a general way the following should be evaluated: Steel Chemistry; Zinc Bath Chemistry; Flux Formulation (if one is used); Passivation Chemistry (if used); Post Treatments (e.g. waxes or lubricants); any other parameter which with production is likely to change (outside normal ranges).
I think if you post more information on your process, you may get a response from a more knowledgable person on this subject.
In the past several years steel chemistry (due to recycling and steel making changes) has gone "crazy" and has adversely affected many, many galvanizers.
Regards,

Galvanneal sometimes has the unfortunate effect that the coating is laminated, and the top of it can separate in working, leaving a flake of zinc peeling off, and a layer of zinc (thinner) left behind.

This might be your problem?