The cabinet doors and drawer fronts are real wood, but the cabinet boxes themselves ... I'm not sure. They might be closer to fiberboard or MDF. At any rate, they absorb a lot of moisture and they absorb ALL THE LIGHT.
The only two cabinets that I hadn't managed to get to with my paintbrush were the ones under our master bathroom sinks -- and a few weeks ago when we had our countertop replaced in there, I had my opportunity.
Here was my cave-like side before:
I'd thought it would be the same quick process the other cabinets had been, but because our plumbing had just been disconnected, there were some damp spots on each side. I had to get some fans out and blow them dry for about 24 hours before I could even start.
Then I just took my leftover trim paint and brushed on two good coats, using the fan to help dry them in between.
Shoulda caulked, too. Didn't.
I still had some shelfpaper leftover from when I did the pantry and island shelves, so I used some of that to finish it off.
It didn't reach all the way to the back, but I don't think anyone will be looking back there with a magnifying glass.
If you've got any dark, dingy cabinets or drawers, I highly recommend brightening them up with white paint. It's so much easier to find things in there, and it's just a whole different vibe in there.