Yes, it's an old trick and can give some extended life from the pin block. Better yet is sand paper wedges. Wet/dry sand paper of 100-200 grid cut into V shapes. Slip the sand paper into the hole with the grit against the wood and screw the pin back in , don't hammer it in. I've read articles that this was Baldwin's suggested fix for a loose pin. But understand that if the piano starts failing all over the pins/multi pins, it's time for a new pin block.Piano tuners today who bother to address this issue are using super glue, FWIW. Don't ask me how it works, I wouldn't dare try it myself. But they claim it's a more permanent fix than wedges of any type. I would think that once this is done, that's it for that pin block. You now saturated the wood pores and grains with a hard glue, it isn't ever coming back to life. But they claim it works.
As I understand it the sand paper is a longer term fix than cardboard and sand paper was at least one manufacturers suggested repair. Both work and do as you suggest, just one way is claimed to be the longer lasting method that was suggested by at least the one piano manufacturer. You hear accounts where shimming doesn't last and others where it lasted for decades LOL. I think it's all about a particular application situation. lets put it this way, I would do what I had to do. Most houses have a piece of cardboard box laying around so it's worth a shot if you have a loose tuning pin. It may only last a short time or may last for decades.There is also a metal mesh shim available, not sure how that one works except obviously it takes up the loose space. I wouldn't think metal to be ideal personally.
I would be happy if this material would work for the benefit of the piano owners. Not long times even
I found it interesting that in the sandpaper technique that you lacquer or varnish the back side of the paper and let that dry over night before cutting the wedges. I don't understand the purpose of that exactly. My only thought is that it must help control the integrity of the backing as the pin gets screwed in.
I would assume that varnishing the back side of the paper done by applying a varnish for better coupling of the pin with a hole
The instructions state to be sure to insert the paper with the grit against the wood.
Soft cardboard shim have hard connect with a hole and a pin. Why that need us using varnishing the backside or it's grit against the wood? That it's creating new friction without a varnishing and a sand
Ya, well I said I don't know the exact purpose of the varnish. You seem very committed to cardboard so my suggestion is to stick with that.