I can comment exactly on the history of paint development, but I can tell you most of the chipping/fading/stains you see on examples such as the ones you posted were not so much a result of "unreliable" paint, as much as they were the result of a lack of emphasis on keeping aircraft looking pristine, especially in the more "rough-and-tumble" areas like the pacific and 1944 Germany. Salt water/air was especially bad for most aircraft paint, and pretty much any actively used aircraft from an area like the South Pacific would begin to show some wear/corrosion. Some air forces, like the Japanese, frequently omitted using primer beneath the paint for expediency, leading to the massive amounts of peeling/chipping you see in pictures of late 1942 Japanese navy aircraft onwards. Also, lots of it is just dirt- all aircraft get dirty, and soot/dust can build up and leave stains. Here is a good article about aircraft paint practices in the Japanese air forces during the war.

I don't know enough about paint to tell you if modern aircraft are any better, or if they are just more carefully maintained. WW2 aircraft were much cheaper and more numerous than their modern counterparts, and many aircraft today require long, hard runways at permanent airbases to operate, which might mean that there is more time for maintanence/repainting nowadays.