I could be wrong, but I don't think that these are meant to be completely water resistant...I stamped some of mine with Stazon ink and colored with Sharpies to use for a party...and just threw them away afterwards.
I would imagine that you could spray them with an acrylic sealer to make them last a little longer, but I think they will absorb more of the sealer than protect the coaster.
Good luck!
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