No idea at all but if you try please let us know the answer.

I doubt it since there is only condensed moisture to deal with. Since I add batting in the middle the batting would absorb the little moisture first. I use coaster more to prevent heat rings on furniture. That is my thought on the subject but I am always curious what others have to contribute

I haven't noticed any water leaking through coasters. But to be honest I haven't spilled water over them, just had a coaster absorbing the condensation from the glass. Interesting question. I'll be happy to know the answer.

I haven't had this problem with any of my coasters or mugrugs. What material are you going to use, this might help with the answer you require. Marian

I don't use vinyl and have not had any issues. Now for a large spill, all bets are off. But also I don't do fsl coasters as I just don't see how they can be absorbent enough. Hugs. Nan w

I have just started doing a two hoop method for coasters using cut away stabilizer and cotton batting. The cut away adds an extra layer of protection against heat and water. For myself it is faster to take it out of the hoop to trim. Then using one piece of wash-away for the outline to set the coaster on for the final satin stitch.

i used a really heavy 'leave-in' stabilizer. Gave them away but I know they get used [and laundered]

I would say yes and the less holes the better. Vinyl on the bottom may keep moisture trapped inside and then you may have mildew or worse mold issues. There is the fusible vinyl. I have used that on top of coasters, but then liquid runs off. Lol! If it is just condensation I think it would be just fine on the back.

I used to worry about this and then I realised that if there was actually enough moisture or a spill big enough to make the coaster wet then the coaster would end up in the wash and not left on the table anyway.

PUL fabric is a washable waterproof product you could try and line or back them with. It is often used to make wet bags or line cloth nappies (or diapers for those of you on the other side of the pond!)

Sorry for spelling errors. Am on my phone