Spraying something like a can of rocker guard on an area like that. The level of buildup you use helps conceal the welds etc, It also will semi blend the repair with the existing undercoating.

I always put a skim of mar glass type body fillers on areas like that. As normal bondo will absorb moisture given a chance.

In a simple case if the protective coating gets chipped in service. As an example. Or backside voids exist as a chance of being a point for moisture to enter the unprotective bondo.

Again I use a mar glass type product as the first coat. It simply is waterproof but is much harder than bondo to work. You probably already know this. Stiil too important to chance someone may not.