My phone case is an "incipio" that is extremely durable but not too bulky. However, it was white, and therefore quickly became dirty. I tried painting it several different colors of nail polish (hence the slight bits of turquoise you see in the picture below) but it came off quickly. I had just bought the lavender Essie and started painting it on. As soon as I did, it began to form these crackles (I have absolutely no idea why). The final product below was how it naturally turned out when I painted long strokes on the case. After the purple dried, I added the gold sequins in some areas. I can't promise you this technique will work-but if you have an old case, it might be worth a try!