Problem: I'm modeling a car and (unfortunately) thought too late about shutlines (panel gaps). Now, as I have my finished exterior, I've tried making the panel gaps by creating them as a separate object and cut them out with a boolean. It works okayish, but in certain areas doesn't make a clean edge, instead very blocky and the reflection is unacceptable when I render it. On the exterior I have the subdivison modifier. I've tried applying the subd and then boolean but it created an enormous amount of data, which was very slow to handle and also, it's then not really possible to modify anymore. Even then, the effect was not 100% convincing. I'm hoping there's an easier way.
Question: How could I do panel gaps that are relatively easy to modify or fine-tune if I want, yet still creating a clean and realistic effect? Are there any tools for that? I just want to somehow project them on the finished mesh, without having to modify the structure.