I will pitch in here since I just had done exactly this yesterday. I had an appointment for ceramic coating (Crystal Sermum Light) and even though specifically asking that the trunk lid not be coated at this time, it was. Oh well. 5 days after picking up the car, I had the appointment for the spoiler install. The service rep was not very familiar with the ceramic coatings but said he will install it anyway, and it will be my problem if it sticks or not. I was more than happy with the proposal.
He thoroughly cleaned the trunk lid and proceeded to install the spoiler. He mentioned that sometimes the spoiler's curve does not match the curvature of the trunk but because the adhesive is very strong it does not really matter.
In my case one side was not perfectly lining up with the trunk and kept popping up about 10mm and this is where he mentioned that this does not usually happen. We tried holding it down for about 10 seconds (pushing hard) but the end was still popping up.
He mentioned that in regular situations without coatings, once you put the spoiler on regular paint, it will be extremely difficult to take it off again. In my case he took the entire thing off twice with ease after pushing it hard against the trunk.
I finally resorted to a lot of duct tape to strap the spoiler and push it hard against the lid. Left it like that for a day and now seems properly attached. I did tug a bit on it and it won't come off.
Long story short it DOES matter if you have ceramic coating. Initial contact and grip will be FAR less effective on ceramic coating. It seems that after a while, with some pressure help, the adhesive cures and grips the coating. Hopefully the spoiler won't come off with time.
