One of the best things you can do is to choose a plywood or wood product that has a lower volatile content (VOC's). These days there are various "green" wood sheet products that are available that mitigate the offgassing problem almost entirely (I believe that these were the subject of a recent Musuem-L disucssion).
The problem with using regular plywood and painting it to seal it is that it isn't really sealed, pinholes in plywwod can actually concentrate more of the volatiles in an outgassing situation. So, if you are using plywood then Marvelseal is really the way to go.
The other factor in all this is how tight your exhibit case construction will be. If there will be ready air exchange then the outgassing will be less of a threat than a case that is almost air-tight, with a plywood product used in it's construction.
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