I had a 26cc Yak that I converted from glow to electric. I covered the huge muffler hole with ultracoat and it worked great. I only had about 1" or less of covering around the edges though; whenever you cover a solid surface you run the risk of bubbles. You can try popping them with a pin and shrinking the covering. There is a chance the covering was not stretched enough when applying though, in which case the bubbles may not come out.