electrophoretic coating is painting by a more complex process than simply spraying but its paint nevertheless. The elecrtic part of the process ensures that the piant gets into every nook and cranny but if MMC then drill holes in the chassis they have breached the coating.The bond between paint and metal is a mechanical one so corrosion can travel down the interface once started.
Hot dip galv is different firstly because the zinc forms an alloy with the iron in the steel so there isnt an interface in the same way. Its almost as if the zinc was welded to the steel - it isnt but thats the best way I can think of describing it. The second difference is that the zinc coating is sacrificial ie if a hole is drilled in the chassis, rather than the steel round the hole corroding as with a paint, the zinc itself is eroded slowly away and the steel remains intact. In effect a little battery is formed with the zinc as the anode. Obviously this wont last forever - there comes a point when the remaining zinc is too far away from the steel being attacked and the steel corrodes as a result.
There is a reason that fabrications like motorway crash barrier are galvanised rather than painted - the reason is that they last far longer that way. Trouble is, hot dip galv is a polluting process and keeping it clean makes it expensive. MMC chassis are now painted not because its better but because its cheaper. But then, most cars on the market are painted and they dont dissolve in days do they.
So paint is amply good enough for a reasonable life of many years but its isnt quite as good as galv. Are you worried about what happens to your painted chassis in 20 years of summer time use?
