Thanks for all the ideas. I'm eager to try a few.

I forgot to add the message about using the vinyl tablecloths.
That sounds like a great idea!

By Jove it works great as a mouse pad!!!!
A new and cute project to make just for mouse pads.

Insul-Bright.

I have used this layering before for non-leaking coasters;
top layer - decorative cotton
inside - insulbright or regular thinner batthing
another Layer of scrap/extra cotton
then - clear plastic from joanns, I can't remember the thickness I purchased.
last/bottom - those waffle weave placemats cut to size or shelf liner for non-slipage.

I quilt the top three layers, {stabilizes the batting} add the bottom two layers and zig-zag, serge, or bind around the edge. You don't want to quilt the plastic layer and punchy holes in it {don't ask me how I figured this one out, LOL}.

Thanks. I wondered about quilting through the plastic layer. Thanks for sharing your experience.

I bought a teflon finished fabric from Joanns its for potholders not sure what they call it,now I just buy a teflon ironing board cover.

I use flannel or wool scraps, sew together and turn out and close hole. Those two seem to soak up the condensation that I get in my basement and in the summer.

I use a square of iron on interfacing along with the batting in my coasters and it works great. I've been making the "folded" coasters in several different sizes for different occasions and the water from the sweating glasses or mugs or whatever has never gone through. They make good quick gifts.

I woulde think a layer of PUL material would work as well. I purchased mine at JoAnns.