The last time I made bottle cap magnets, I sprayed them black. The finished product turned out well but a bit dark, so after my husband's urging, I decided to make another batch of magnets using white spray paint this time. I'm guessing it had something to do with the paper turning a bit transparent when the glue soaked in and the dark background absorbing light instead of reflecting it. At any rate, the white ones turned out even better.
So I sprayed my bottle caps with some white spray paint (cue the riveting photo of spray painting). I like to use a cardboard box to catch the spray. My white spray paint turned out to be a rust corrosion variety (I'm guessing I didn't read the can closely when I bought it), but it turned out fine, a bit textured, but still did the job.
Next I found a bunch of images on the internet that would shrink down to a one inch square or circle so that they could be punched out with a circle punch. Since these are for personal use, I just found whatever images I liked, but if you hope to sell them, you'll need to create your own images. I printed the images onto some matte photo paper and used my 1 inch hole punch to cut them out.
The last time I made these I had problems with the paper circle images curling up when I put the diamond glaze over the top of the image, so I glued the circles down using a different glue (tacky glue worked ok) and waited for it to dry before I put down the glaze. I learned from last time that applying the glaze in a circular motion without lifting the tip of the glue works the best. You end up with far fewer bubbles. Large bubbles can be popped with a toothpick and small bubbles can be tapped out (but they are stubborn). Once everything is bubble free, they can be left to dry. The bottle caps above had been drying for about 8 hours and they were still a bit cloudy.
A few hours later they were much clearer. So allow yourself at least 12 hours of dry time (and longer if it's humid).
Next I applied some 1/4 inch neodymium magnets to the back of my caps with some E6000 glue. I'm sure that hot glue would work for this step too. The magnets are attracted to the back of the caps, so you may have to use a toothpick to move glue once the magnets have squished it out.
After the glue was dry, I hung them up on my fridge. These look so much clearer than the black ones I made, but they coordinate quite well, I now have a huge set of geeky magnets to use for years to come.