Hi - first time framer here and I have to do an oil painting. I have NO idea what to do. I bought "canvas clips" - they didn't have a size option so I thought they were one size fits all. How do I use these? Are they for metal frames or wood frames? Still debating whether to frame it with metal or wood. The canvas is already stretched (don't painters normally paint on a stretched canvas?) and it's on bars that are 2 1/2 inches wide. Should I just put a foam core backing on it, seal it with points and backing paper and forget about these clips? Thank you!