I coated mine mostly to make them easier to clean up and disinfect after a hatch.
Get the inside of the bottom half as clean as possible; I even used disinfectant water on mine after gently scrubbing the entire inside with soapy water and rinsing. Find a place where it can sit in the sun for several hours to get completely dry. That will also further insure it is "free" of germs.
Buy a spray can of LATEX primer.....very important that it is latex. Any other kind of paint will react with and eat up the foam. Give the inside of the bottom half and the rim all the way around at least a couple of coats, drying thoroughly between each coat.
Buy another spray can of paint....doesn't need to be latex since the foam is now protected. Mine was the enamel intended for metal lawn furniture. Put on 2 or 3 coats of this finishing paint, drying thoroughly between each coat. You can now easily clean with just a rough cloth or foam scrubber after a hatch. I still disinfect thoroughly in addition.
I have heard of someone using vinyl caulking....the stuff that comes in tubes for fixing leaky windows, but have no idea what kind of prep work you would need to do, or if that stuff might eat the foam of the incubator. I believe they spread it evenly around the entire bottom half of the incubator with a putty knife.
One thing you need to do when using anything to coat the incubator is make sure you DO NOT fill up the ventilation holes in the bottom half of the incubator. I inserted 2 inch lengths of vinyl tubing (available at stores like
Tractor Supply) to protect the holes (with wooden matches in the tubing to keep it from filling up with paint...removing the matches when done painting), and the process has worked fine.
Hope this helps you. Have fun.