"Pentimento" is an underlying image in a painting...perhaps an original painting, whereas "palimpsest" means a manuscript where there is some underlying writing that shows through. However, another definition for "palimpsest" is "An object or a place whose older layers or aspects are apparent beneath its surface." Hab did discuss palimpsest here previously.

Can the term palimpsest be used to mean a pentimento, but not vice versa?

I'm in One of Those Moods Again.

Jose, born poor in Sacramento,
Discovered a strange pentimento.
His life was in flux,
Now he's in the big bucks
Leaving Jose today quite contento!

A palimpsest is a parchment, usually sheepskin, which has had the original writing scraped off so it can be reused. It may or may not have new writing on it, and, depending on how good a job was done on the scraping, it may or may not be possible to decipher what was originally written.

A pentimento is a canvas that has, at some stage, had the original painting painted over, and, usually because of age, some of the original painting starts to show.

Since they refer to quite different things, the term palimpsest cannot properly be used to mean a pentimento, or vice versa.

Okay, arnie, so here is where I have difficulty with dictionaries. That second definition doesn't indicate a material at all. Why is it even there then, if it is incorrect?

Get a proper dictionary?

Okay. This weekend then I will check the OED. I assume you consider that proper?