Do not peen the mosaic pins. Fill with epoxy.
A short how-to if you want to make them yourself...
I use a handheld automotive vacuum pump. I have a Mity Vac, but you can get a cheaper one from Harbor Freight.
Use clear fish tank tubing piece, a few inches, between the mosaic pin and the pump. Then a short piece on the end of the mosaic pin.
Your mosaic pin should allready be stuffed with little pins.
Mix epoxy and colorant (I use Devcon 5 min epoxy). Colorant available from knifemaking supply co. or paint company.
Stick the tubing in the mixed epoxy, give the vacuum pump a squeeze or two. You'll see the epoxy getting sucked into the pin, that is why you use clear tubing. When it starts oozing out the top end, cut the vacuum and let it set.
You are done.
Peening them will only destroy them, you have to secure with epoxy.
Dan