I have had to repair all 7 of the pins I got. Each one had the studs soldered in a very poor way. Lots of cold solder and lots of soot.

I use soft No.2 Solder from a jewlery and Lapadary shop. It melts as a temp. far lower than the rest of the pin. It is used by silversmiths when they want to attach fastenings and don't want to re-polish the front surface.

Use lots of flux. Use a needle flame butane torch. Place the pin on a heat sink to prevent blistering the varnish on the good side.

I did all seven kilt pins in about an hour and that includes first removing the original solder joint, prepping the surface with sandpaper, and cleaning afterwards.