One of my favorite tools in the studio is old credit cards.
I prepare them by sanding one edge with 400 grit
wet or dry sandpaper. ( you can use an emery board or fingernail file too). I sand them wet to keep any heat created by friction on the abrasive sanding paper from deforming the plastic. This smooths them and removes small burrs and irregularities along the working edge left over from their previous life. They have small nicks and rough spots, as well a some that I create using them. You can have a small bowl of water to set them in during a work session. This keeps them both clean and ready to touch up with the sanding paper.
I use them as glue spreaders, or to lay down a smooth layer of gesso.
I use them to squeegee out a water-soaked paper.
Lay down oil paint background into a fresh canvas.
Clean off a palette knife.
Paint shield for straight edges.