Hi, Geoff. I used to work in a fabric & craft store, so I learned some things about glue.

Natural materials (wood, wool, cotton) are easy to glue. They are porous and absorbent. Man-made materials are harder, and you have to read the directions on the glue and look for the right one.

You probably have an aluminum bar in your autoharp, and that's going to call for a glue which bonds to metal. But your glue also needs to bond to the felt, which is probably synthetic. Also, your glue must not dissolve the felt. It must not bulk up when being spread, and it also needs to be removable when today's felt needs replacing.

It's true that Bill D. likes the felt strip he rescued on the job, but maybe he's not as particular as you about the way his autoharp sounds and acts. It might also be that the felt product he's been using has changed since he plucked it out of the wastebasket.

I think that given the things that could go wrong, you should order replacement felt specifically made for an autoharp.

There are YouTube videos that show you how to do it.