Making marbled phone cases are really fun and pretty easy. You can use the same technique on drinking glasses (to the bottom of the outside of the glass), candle votives, etc.
You will need:
-item to be marbeled
-a disposable container to fill with water
-water (I had better luck with filtered or distilled)
-nail polish (3-4 colors looks best, I like adding one metallic for more dimension)
-fine grit sand paper (optional)
-foam brush to apply Polycrylic sealer coat (optional)
I recommend starting with a hard white cell phone case. Colored (or even light neutrals) pop nicely on a white backdrop and the rigidity of a hard case keeps the nail polish from cracking. I bought mine on Ebay for only a few dollars each.
Fill 3/4 of your container with room temperature water. Working quickly, begin dropping drops of nail polish right above the surface using your nail polish brush or by pouring directly from the bottle. Add additional colors in the center of existing drops.
Use a toothpick to swirl nail polish drops around on the surface until you have a pattern you like. Hold phone case with the side to be marbled facing down. Dip phone case straight down into the water, this will transfer the nail polish from the water surface to the phone case.
Still holding the packaging tape handle, turn your phone case over to see your marbled design. The nail polish should dry very quickly. Use a paper towel to dab excess water off the surface. Use a toothpick to pop any bubbles that may have formed under the nail polish.
If you don’t love your design, use some nail polish remover to wipe off your design and start again. I’m a perfectionist, so I redid mine several times before getting a design I loved.
Once your phone case has fully dried, add a few coats of Polycrylic to protect your marbled design. 2-3 should be enough. Make sure to get the edges of the case, since this is where the case gets the most wear.