da Rock's got it. The 30 minute is more penetrating into wood and other porus materials. It "wets out" better. It also has longer to link before it kicks and you get a more consistant result. Epoxy worls because the molecules form long chains. The longer it is "free" the longer the chains and the stronger the bond. I don't use anything faster than 30 minutes. Some of the West System epoxy I use is "120 minute". At that rate the amine waxes have a chance to float to the surface and you get a glassy result when hard.