That what I am talking about... Not sure you call it in English, but when they paint it and good body shop they use some material to make it smooths like a glass and then painted...this way is do not look waivy

 

Even urethane body pieces need some prep work before installation, but fibreglass Renntech and Brabus spoilers will UNDOUBTEDLY need moderate to majpr prep work in order to achieve a perfect fit/paint finish.

90% of a good body mod is the prep, 10% actual painting.

When I had the Lorinser decklid spoiler installed on the S430, my bodyshop guru filled, sanded, filled, sanded THEN painted. Same exact process with my urethane AMG ducktail for the W124.

Do the work and it'll be worth it. The "purchase-paint-slap it on" method should be left for honda bodykits.