There is probably a leak somewhere if the resonator length is correct. The leak could be where the plug meets the inside of the resonator. Paint won’t matter. Aluminum tubes will make the bars louder.
Test the pitch of the resonator: hold you hand on the bar as you strike it, you should hear the pitch of the resonator, then compare that to the pitch of the bar. It’s nice having an adjustable resonator plug so that you can move it up and down until it sounds best.