The point of origin for the mount to crack (the fractured spot-welds are a separate problem). While the above might address one (1) of them, it won't re-stamp the differential mounting bracket to remove the popular stress-riser that BMW graciously built into the part. However, there are two (2) distinct and separate areas of failure, almost universally addressed, and repaired/reinforced at the same time. Had the car been manufactured last month, with minimal exposure to surface oxidation and (dirt) contamination, it might be possible to impart enough strength into the hollow XMBR that supports the trunk floor to make a difference using an expanding foam. Ralph describes it above, and I will reiterate: for bonding methods to work, ADHESION is key. But it is an interesting thought and worthy of consideration.

Those approaches seem to me to be more akin to the 'put a screw in it' or 're-weld it' solutions that are occasionally debated here. However, that solution, whatever it might be, does not include simply applying some off-the-shelf epoxy to the already welded together and painted surfaces of our trunk floor, cross member, and differential mount bracket, or simply applying some epoxy to broken spot welds. It would be a lot of work (read expense) to engineer and test an epoxied solution, but in the end it could well be a better approach. Another advantage to an epoxied solution is that the range (and weight) of reinforcement materials is greatly increased-Kevlar, aluminum, carbon fiber, glass mat, etc. There is also the question of how the epoxy bond would hold up to temperature extremes and body flexing-but that is more a matter of polymer science than a limitation. to bare metal-and roughing the metal surfaces being bonded as such, an epoxied solution might involve greater preparation then the current welded in one. Of course, any epoxied solution would necessitate removing all paint on the surfaces to be bonded-i.e. I can imagine some advantages to that approach: heat welding necessarily changes metallurgy, and can promote brittleness and corrosion while epoxy could provide a stronger bond by being applied across the entire surface area, rather than just at plugs or across a bead. In that regard, an epoxied reinforcement solution could look very much like Randy's kit-only epoxied in rather than welded. Whether welded or epoxied, the question is less what method of panel bonding is used, than what panels are being bonded by that method.

Different epoxies have different qualities, strengths, and weaknesses, and can be very specifically engineered to particular applications. However, it is simply a different method of bonding materials together-neither better or worse than welding, but different for different manufacturing processes and purposes. Epoxy panel bonding is common place in auto manufacturing today.

E46 m3 subframe reinforcement epoxy