WUNSIE wrote:surely it has something to do with the plant in which they are painted
Basically, this is it. Sometimes a contaminant can get on the body after or even while it's dipped. Not visible, no fisheyes or anything, but just hasn't stuck and only becomes apparent once it's damaged, even just a bit. A shift in humidity can be enough and Thailand is very humid.
Back in the late 1980s I had to deal with hundreds of Falcons that had a racing stripe right down the middle. Only visible in certain lighting conditions, happened because one of the spray heads in the factory at Broadmeadows had an almost microscopic dot of dried paint on the aircap. That was just after Ford spent something like $8M upgrading the paint facility.
The single coat of clear on door jambs and similar is intentional. It's all that is necessary, given that those areas are not exposed to UV like the rest of the outside. However, the thin clear coat issue on the rest of the outside is another story. That's just a way to save money. Holdens used to have a paint design life of 10 years, Thai built stuff (except Honda) is 5 years. Toyota saved over $100M USD annually a few years ago by reducing their DFTs by only 30μm. Honda did the same thing, with little effect on longevity, but kept their paint quality standard high. Unfortunately Mitsubishi went the whole hog and (IMHO) compromised on quality as well, more in Thailand than anywhere else. My Pajero, built in Japan, has almost double the paint thickness (at 130μm) of a Triton and after nearly 10 years of living outside every day shows absolutely no sign of degradation. But my snorkel, on which I tried a cheaper clear that the paint company was trying to sell me, is almost back to bare plastic. So quality matters, even if it comes at a higher cost, it's always cheaper than having to respray it.