Originally Posted by Hujan
You have to make sure it's stretched thin enough so there is no gap between spoiler and trunk.
When I investigated using the 3M tape I recall finding instructions from 3M stating that you should not stretch the tape when applying; stretching it reduces adhesion. If you look at the stock spoiler, the tape is not stretched at all and the spoiler sits above the trunk slightly because of the tape. 3M does make the tape in different widths and thicknesses. Did you get the thinnest tape possible?