I need to patch a hole that rusted thru some sheet steel on the underside.

I can get to it from above.

I'm thinking of "gluing" it in place using the kind of butyl rubber sealant made for rain gutters, because I have some on hand and its probably the same stuff used to seal windshield glass in place...

What I want is something that will remain flexible against vibration, so the patch stays in place.

I think this would be better than Bondo -- also I think Bondo is somewhat porous.

This does not need to be painted. I could even use a piece of plastic instead of aluminum for the patch...

I will prime the outside edge of the hole & put some kind of topcoat on it because you can't leave primer exposed... color is not an issue. Then maybe I'll glop some more butyl over the edge just to help keep water & grit from abrading the area.

Does this sound reasonable?

 

Always try to cut away the rust then rivit another piece of metal there. You can then seal it with undercoating. I use to work in a body shop before they made plastic anything for cars.