I was aware that there exists such a thing as a wet palette, but had never been inclined to use it as I had formed the impression that it was mainly used to enable the painter to easily mix two colours in various ratios and have all the mixes available while painting and one of my most important army painting shortcuts is that I never mix paint anymore (using the Foundry system helps in that regard, but even in my Vallejo days I had my no mix colour combinations).
A wet palette is basically a palette that somehow (capillarity, permeability and possibly some osmosis actually, if you want the scientific explanation of 'somehow') automatically adds water to the paint on it (hence the 'wet' bit), in a hopefully controlled fashion. There are undoubtedly hideously expensive 'professional' versions of this, but the basic DIY version is a plastic box with something moist (a sponge, towel or kitchen towel), covered with a semi-permeable layer like grease or baking paper. You put your paint onto the top layer, and water wicks up into it and dilutes the paint. If you get the amount of moisture and your type of semi-permeable layer right, the rate of evaporation of water from the paint and the rate of water flow (I guess 'seepage' would be a better word) into the paint reach an equilibrium which keeps the dilution of the paint at just the right level for your painting style and method.
So far the theory. Here's how, on a whim, I decided to put it into practice:
My wet palette matches the above DIY description to a lick. I took a plastic box (fake Tupperware - nothing but the best in my household), added some moist (not dripping wet) kitchen towel and put a single layer of baking paper (which is normally used to put cookie dough and such on before chucking it in the oven) on top. And that's it. Here's a picture of the kitchen towel underneath (with a spot where I accidentally poked through the baking paper so paint leaked through):
And it's really as simple as that. Although I never saw the use for me in this before, I have now become a fan. I can now easily paint just a little bit at a time without having to waste paint (as it used to dry on the palette when not fully used, and once paint is on the palette, you can't put it back in the pot) and when I close the box with its lid, the paint stays usable for days (unless I poke a hole through the grease paper - then the paint on the grease paper there dries out quicker because you lose the semi permeable layer which controls the rate of seepage). I do occasionally have to add some more paint to control the dilution a bit, but it is a lot less than having to add water every few minutes. So count me converted.
One final note: in the first picture it is obvious that the paint tends to seperate into pigment and solvent (the grease paper and the water act as a sort of thin film chromatography machine) when left overnight. This is not a permanent seperation however - a quick stir with a paintbrush quickly mixes it back to a usable state.