Most playing cards are coated in plastic. If you could coat your cards this may create a unifying surface. Make a test card. You can also lightly sand (with a really fine grit) the painted surface once it is completely dry this way you won’t feel the paint through a coating. A spray-on coating will apply more evenly. Test sprays, some have a bit of a tacky finish, you don’t want that. I have used clear wood sealer that you can find with spray paint because it is meant for more handling than a sealer you could get that is specific for 2D art. Good luck!
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