I haven't had that problem, so probably can't help with that. I have had one on my desk about a year now and it's never been washed after the first time. (How do I make an embarrassed face?) It only has a water glass that sits on it, though. Mine has a double layer of plastic from a disposable grocery bag.

The ones I made for myself are made from pre washed cotton fabric for front and backside with batting in the center. I never added any plastic and had no issue with liquid soaking all the way through I wash them throw them in the dryer and they are not harmed.

I make mine with felt on the top and an adhesive foam sheet on the back. I clean the adhesive off the embroidery needle between coasters and have had no problem with stitching them. They are waterproof and wash well but take awhile to dry (flat). I don't use any batting or plastic in them.

I put iron on vinyl on the top of mine and have not had any issues.

I have made mine with cotton on top warm & natural in the middle & cotton on the backside. Never had a moisture problem......@least not yet. I have also made them using flannel backed tablecloths cotton top warm & natural in the middle flannel backed tablecloth on the backside. they are great for outside in the summer time when the glass has a lot of condensation. I wash by hand & hang to dry & they are just fine this way too. ~hugs~

The ones that I have made were felt on top, stabilizer in middle and felt on bottom; however I do remember reading once that a site recommended using table plastic like table cloth fabric that a cotton flannel type on the underneath side. Joann's carries it. I bought a remnant once to try ,but i never did try it. Maybe your daughter just need to wash them. She may have been afraid to wash them. Kay

I have also made them with felt i do not use stabiliser, when the designs finished on top, slide another piece of felt underneath and then finish the edging. Back of stitching is sealed with the second piece of felt and you only have an edge on the back. They wash very well just be careful of the colour you use as some colours run. Hope this makes sense to you and helps. Sandra.

Have not had issues with mold, but I haven't used the vinyl from an old shower curtain either.

If the concern is to protect the table from drips/moisture, perhaps you could use a piece of flannel backed vinyl. That would also add a bit more padding for absorption.

I use two to three layers of felt . If you are concerned about the water getting through you can buy a rubber material that they use for baby things. I also use thick canvas drop cloths from the hardware store they hold up nice and wash well. Suzanne

It is caused by the coaster not drying out fully before being used again. I have had that problem with a mug rug that I use. It does not have a water barrier in it, but it would get wet from the cold drinks I put on it. To make a long story short, it did not have a chance to dry out and it got mold spots on it. The mold was mostly where the stitching was dense as it takes longer for those areas to dry out.

When I make coasters, I do put a water barrier in them. I use table cloth vynal with the flannel backing in them, flannel side out. Unlike the mug rug, they do have a chance to dry out between uses.