Hi, I have a mugen rep wing fitted to my mk8 5door and this was pre primed just one coat though when i first brought it. I took it to a body shop to get it primed again and prepped properly and sprayed i was able to get information out of others (sprayers and wrap guys)(i work and study in the industry of motorsport/motor vehicle so am able to get information quickly from buisness and friends and family ) the wrap guys were completely honest with me and said that the wrap doesnt always stick well onto the primed surface and if it does they arent a massive fan of the finish it leaves so in their honest opinion it was best to be sprayed anyway so i had mine sprayed. The wing was fitted on by the bolts and so on but I also used windscreen bonder to hold it on so that either way it wouldnt move anywhere (suggested by my cousin who works within bodywork) however i have heard many others use things such as tiger seal and sikaflex as Dazvti suggested above. hope this helps with your decision 