After searching for a while, I found a great coating idea from this wonderful blog, and decided to give it a try.
I created several designs using my collage works and printed them out on the Shrinky Dinks sheets. The colors on the sheets have to be adjusted to kind of "faded look" in order to maintain the original colors after shrinking, so I used Photoshop to modify the colors 50% less intensity.
I needed to punch a hole to make a charm before baking.
Curling and shrinking....
I covered top of the each piece with UTEE (Ultra Thick Embossing Enamel), and arranged them on the same cooking sheet covered with a parchment paper.
I decided to give them another UTEE sprinkle and bake in order to make the top of the pieces look smoother.
Voila! The top is smoothly enamel coated.