MIMF

I have a “red label” Yamaha FG360 in for re-attaching the pick guard. It’s a sort of humming bird style pick guard made of plastic. I had re-attached it with double stick tape a while back and needless to say it didn’t holdup. When I first worked on it the owner had years ago attached it with what looked like plastic model cement, the stuff I used as a kid putting together those plastic airplane and car models. At any rate the old glue had partially melted into the plastic in places and appeared to have softened the finish enough to have it squeeze out from under the pick guard leaving a raised area of finish out lining the pick guard. I sanded and scraped the back of the pick guard to remove the old cement and clear it up. The tape stuck fine on it. I’ve scraped the body under the pick guard and put a couple coats of shellac to even out the finish and cover bare wood. So I guess my question is, should I re-attach with tape again, or is there something else that would work better?

Is the guard really flat, or is it cupped?

It needs to be flat if tape is to be expected to hold it.

When you scraped the back of the guard did it smell like camphor? If it did then it is most likely made of celluloid and can be flattened.

Just to extend what Michael said:
If you are mating flat surfaces (flat guard to flat top), then the tape will hold it on very well. It sticks well to both celluloid and shellac.