Of course you could buy a wet palette at your arts supply store - but you can easily make your own.
The wet palette consists of a closable container, a sponge and a mixing paper. You fill the container with water so that the sponge is well soaked, then put the mixing paper on top of the wet sponge (press it a little down to stop the edges from rolling up).
Now you can put your paint on the mixing paper, thin it (you still have to thin it though), mix it, work with it.
The water lost through evaporation can diffuse through the paper and keep your paints wet, but the paint can't seep through the paper.
For our custom made palette we can use a big blister box (e.g. from Warmachine or Fenryll) or similar, something sponge-like (e.g. dishwash towels) and baking paper.
Here's a shot of my wet palette. I taped a Fenryll blister to an unused CD jewel case cover to give it some stability (spilling its contents over my table wasn't my intention). The 'sponge' is made of several layers I cut out from a new dishwash towel - it was blue, but that doesn't make a difference. Make sure you leave some free space between the container's sides and the sponge.
I made a cardboard template for my baking paper, so I always get the perfect size.
Water evaporates, water needs to be refilled. Now the extra space you left when cutting out the sponge comes in handy. You can fill the water into it and easily check the water level so that you don't flood your paints. Once the lid is closed your paints will keep wet for several days (and probably even longer).
Make sure you clean the palette thoroughly from time to time to prevent the excessive growth of microbial life.
You can throw away parts which have done their service because everything's cheap.
Now go ahead and make your own wet palette.
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