Originally Posted by Alan@TreeFort

Spoke prep goes on the spoke threads before the wheel is even laced - it is the first step. There is no way to apply it after the wheel is built. As shown in this thread, its not necessary to use spoke prep. However from my experience I have found that it helps building the wheel, especially for first timers. It makes turning the nipples easier at higher tension because it decreases abrasion between the threads of the spoke and the nipple. In my opinion, wheel adjustments actually become easier with the aid of spoke prep. I have built about a dozen wheel sets at this point - I've tried building wheels a couple times without spoke prep and while the end quality of the wheel build was still just as good, the time put in and adjustments needed were definitely increased.