Hi Woojim, sorry about the late reply. Information regarding paint and its effectiveness as Carpenter Bee control has been pulled from numerous sources, some of which may include information discussed on university extension offices websites. But here is a link to a Georgia Department of Agriculture website: http://agr.georgia.gov/00/article/0,2086,38902732_0_41318134,00.html
Why does this work? I don't know. Does urethane work? Possibly. It's difficult to find the answers to these questions because there isn't a lot of research done specifically on what chemicals will stop Carpenter Bees. It's a little unrealistic to expect people to do this for you. Why not give it a shot yourself and tell us what you come up with? I'd be interested to hear the results of your own study, yes?