|I removed about two tubes' worth of dried paint off this palette.|
I think it's a point of pride among studio painters to leave thick slabs of dried, leftover paint on the palette. If your palette covers enough acreage, why not preserve these coprolitic remains? You have worked hard to get where you are today, and these are the trophies to prove it.
But for the plein air painter, not cleaning one's palette thoroughly can cause problems down the road. First, the mixing area on most palettes is small. You can run out of room fast if you don't keep things tidy. Second, dried-up paint adds weight to your gear, and if you like to hike a distance to your painting location, you want to pack like a long-distance hiker, for whom every ounce counts.
I'll admit—I'm a terrible one for keeping a clean palette. As the above photo shows. It seems that my mixing area just gets smaller and smaller. It's the Incredible Shrinking Palette. But now and then, I'll take out the gloves and the putty knife and scrape off those fossilized piles as best I can. It's not a pretty process.
Palettes basically come in three flavors, and each requires a different approach when cleaning up at the end of a painting session.
Wood palettes. These can easily be scraped clean of wet paint with a razor blade. But you don't scrape it by pushing the business end of the blade forward. That would just cut into the wood, ruining it. Instead, you drag the blade across the wood with just a little pressure. This takes off most of the paint, and a little OMS on a rag will finish the cleaning. If you forget and let the paint dry, you can apply a putty knife to work off the big hunks and then use a sander to get back down to the wood. To help with clean up, make sure you prep a new wood palette (or a newly-sanded one) with linseed oil first.
Acrylic palettes. You don't really want to use a razor blade on these, as you will gouge the surface and make it even harder to clean. Instead, use a plastic squeegee tool to scrape off the wet paint. Follow this with a rag dampened with OMS. If you make the mistake of letting paint dry, you can take a putty knife gently to it—but that's about as far as you can go. I really don't like acrylic palettes for this reason, but they are much lighter than the next palette.
Glass palettes. These are the easiest to clean, wet or dry paint. Dry paint just takes a little more effort with the razor blade. Unlike with wood, you push the blade forward. But the glass can be heavy, and it can break. I love my glass palette for the studio. But for plein air, I prefer wood.
I should mention one positive to leaving those piles of paint: They serve as a memory jog for where to put out your different colors.
Honestly, I'm not fastidious about cleaning the palette other than the mixing area. I do try to keep the mixing area clean enough so I can properly see the color of my mixtures. But as for the areas where I lay out fresh paint, if there's a pedestal of dried paint, I just put fresh on top of it. I don't clean these areas until I have trouble closing the lid—or need to worry about weight.