Urethane (or the poly kind) is poly-ably not the best for your motherboard as is subject to degradation by ozone and a host of other things found in computers. Also, since (poly-)urethane hardens it is subject to cracking after long term thermal cycling. What would be better is flexible silicone. I was reminded by the guy at http://www.benchtest.com about what we used to do to waterproof electrics on helicopters. Visit his site and scroll about 1/2 way down the front page and you will find all the answers you need young Jedi..
i just coated my new mb with silicone conformal coating that i got at http://www.cir.com/ this stuff is made for coating circuit boards. when spraying it on, make sure you mask all areas you don't want coated. cover anything that plugs into gets plugged into like your cpu socket, dimm sockets, agp and pci slots, all jumpers, ect. also make sure you cover the metal grounding contact holes that ground your mb to the chassis. also, try to spray this stuff outside as the fumes are dizzying (unless your into that) and have some real staying power.