"Elle" wrote
Unfortunately, as I prepare more and more of my home's exterior surface
for
painting, I think I am finding a 'home construction deficiency' of a kind.
Some of the trim seems to be made not of wood but something like a thick
cardboard-y wall board.

Correction: There is a definite, pronouned grain pattern in these trim
boards. Knot patterns are on them. Yet it still looks like highly compressed
cardboard on the sides where the paint has come off. Plus, like 99% of the
wood siding is in good shape (apart from needing a nice paint job), but only
about half the trim is in good shape. The old paint peels more from the trim
than it does the siding.

Maybe because the edges of the trim are exposed to water, so it soaks the
water up more readily?

All experience welcome.