I am relatively new to fused glass and have been making some pendants and coasters. I often have a base layer and then, particularly for the coasters, a 2nd patterned layer of multiple colors & shapes. Is it necessary to put a 3rd layer of clear on top? I know the magic thickness is 6mm but much of my glass is 3mm so with the two colored layers, I am already at 6mm. If I DO add a 3rd clear layer to bring the total to 9mm….is that too thick?