I just got a roll of this, and was thinking it could give me some trapunto effects without the work. Has anyone used it this way?

Can I leave it in the quilt to dissolve on its own, or should I tear it away? It looks like the regular tear-away, not the clear gelatin products. I wanted to do some thread painting and tearing it off may not be easy in spots.

I think they just suggest tearing/cutting away the excess.
The rest can be left in... You may want to soak the finished quilt for a bit, this can take a tad longer to dissolve :D:D:D

I have used batting along with wash-away thread for machine trapunto work, but have not heard of using tearaway/washaway interfacing for this. How would it be used?

What I was taught with the batting is to use a high loft poly. Machine sew the top with high loft poly underneath, using washaway thread for the pattern. Cut away high loft poly outside the trapunto design, about 1/8" away from the stitching. Layer this top with batting and backing in traditional way, then sew around washaway thread lines with regular thread.

there are so many out there...even some like Ricky Tims brand that can either be left in or torn out. If left in, it loses it's stiffness but retains a lightweight and soft layer that you don't feel between the fabrics.

My idea was to use a printed fabric for a mermaid's tail, embellishing it with fancy thread through a layer of batting and the interfacing. I am hoping that when the interfacing dissolves, the extra batting will "puff" out around the stitches, giving extra dimensions to the details.

My main concern is that the interfacing, being fairly thick, will not dissolve entirely. The background will be deep blues, and any white bits will distract.

I guess I should make some samples and see what this stuff will and will not do.