I haave used dried coffee grounds for years. Of course you could buy a coarse grind and sift out the small particles and make your piincushion. Your pins will not rust. I have had the same pincushion since I was 14. We were very poor and the school taught Home Economics to *th and 9th grade students. I made my own pincushion and haave recovered it many times since then. I am 67 years old. Some years I need to add a few new grounds. I cut two rounds and double sew them together leaving a small opening. I make use a funnel to stuff grounds in side. I sew it up and starting in center, I sew from top middle to bottom middle, up around the side to the top and back down around the side to the bottom and up to the top until the cushion is sturdy.