—”I have some jeans I’m thinking about repairing that way.”
One of the many wonders of wool is the micro scales on the hair fibers. This makes for easy spinning and felting.
If you have ever had the misfortune to untangle a bag of springs, similar. And adding warm water and something to make the fibers slippery like soap and massaging them they work tighter and tighter together. Also called shrinking.
All wool is not the same and some wool is treated to shorten the scales for less itch.
The problem is cotton has no scales.
Yes, you can add an under layer for applique, but I’d be careful about a structural repair.
Wool will shrink with washing and pull away from the denim.
I’ve wet and dry felted for 40 years.
I may have to stitch around the edges of a cellulose/poly patch by hand, but I’ve been needle felting cellulose for a few months, now and it is responsive.
I did a needled wool patch I wet felted and then needled on to acrylic felt that has held up after hand stitching to denim, but it is showing wear after several wash/dry cycles.
I have also been working with silk on wool on acrylic needle felt for several months (acrylic won’t distort with working, the wool provides a substrate, the silk is translucent and reflective and it also tends to form long straight strands). Very strong, too. I don’t wet felt these as it flattens the effect of light I am after. The cellulose handles like silk and is also quite strong. I know the polyfil will really compact if I use a multi-needle tool.
All non-wool fibers do dull the needles quickly, though.