Hi can anyone recommend a undercoat that will stop the corrosion on boot lid getting any worse
My mk3 had the same issue.
I took mine back to bare metal and used std paint primer.
rust remover
sand,scrape, wire brush back to as much bare metal as poss
rust remover again
primer
paint and breathe and relax.
then, probably, do again at some stage in the future
I’m assuming you have a soft top as like mine the boot is aluminium so stripping back to bare metal with wet and dry is sufficient (no rust remover req)
Mazda skimp on paint in the interior panels so after you have sufficiently sanded back to metal and applied a good quality primer and several coats of top coat paint (special attention to edges around number plate light apertures) you won’t have any more problems.
On my mk3 after I sorted the blow in aluminium boot lid I use to leave grease in that area and under wheel arches