The penny trick seems to work pretty well for me. I have a commercially produced wet palette, and the lid is NOT completely air-tight. So if people are using more air tight storage containers, maybe that is increasing the microbe production in some way?
Interesting to think about. The three wet palettes I use, their lids aren't air tight either. One's a plastic box with a hinge that my airbrush shipped in, closes just fine but doesn't 'snap' shut or seal tightly, just kind of lays closed. The other small one I use is basically the same thing, just a little plastic box with a claspy thing in front that keeps it closed and finally my big one is this big old metal box that I mentioned was leaky due to a faulty seam. The lid for that one just lays on top and I can press it down to seal it, but the seam isn't properly welded so it's not airtight.
I just looked over at the two smaller ones as I typed this and they're both now dried up with the paint I left in them, along with the sponge, completely dry since I haven't used them in weeks and it's been blazing hot here. There is literally no odor at all from them when opened. Nothing at all. Could be due to climate as well...it's just dry and hot here about 8 months of the year, haven't had a day below 90 in about a month and have almost forgotten what clouds even look like, saw a contrail from a passing plane this morning and that's about as good as it gets lately.