Body Part Gap Filler?

[Rein]

I had an issue where I needed to remove a rear light mount. This is the plastic or composite part that fits into the panel through which the light fixtures are threaded and to which the actual light fixture itelf is mounted with 2 screws. It was steel-pop-riveted in and had some kind of white filler to plug all the gaps between it and the metal body panel to which it was mated. I’ve managed to grind all the rivets off (new tool, a Dewalt angle grinder model DWE4120 - love it!) and clean up all mating edges but want to make sure I use the right stuff to bed the mount to the body panel. The stuff I cleaned out is still actually somewhat non-brittle - amazing after 15 years. It doesn’t seem particularly adhesive, although I did have to use a large flat-bladed screwdriver and mallet to remove a few larger pieces here and there - they just basically popped off with not a lot of resistance. I’ve never worked with Bondo but I suspect maybe this is what was used. Most gaps are a bit less than 1/8th inch but in a couple places there were thicker applications filling gaps in the range of 1/4 to 5/16ths.

[WE7X]

IMHO< you do not want BONDO. It is brittle, like all regular body fillers.
I would suggest either going to an automotive paint store ( NAPA may have it, or could get it) and ask information for body seam sealer ( your application may not be right for this, but it will get you and the clerk on the right track).
Body seam sealer is usually applied where two panels overlap, and applied on the inside of the seam, so it might not work for 1/8" gap filling but a specialist could probably direct you to the correct stuff.
What you are finding is, I think, maybe the same stuff that is put between the roof rafters and the roof panel. It is white, somewhat rubbery, and acts as an adhesive/cushion and sound deadener.
I think most body shops would be able to direct you to something that would work properly for that application, or you could try something a simple as a good silicone caulk.
Best Wishes.