I seem to remember from my college days that the metals where the weld takes place have an electrical reaction that will lead to eventual corrosion formation.Noble and ignoble as I recall.
Shoot me down in flames people...
but it was 45 years ago
However it is of more significance that repair is sealed from Oxygen and so air as early as possible.
During the repair it is important to ensure that as much of the ground back area is sealed before welding takes place and all rusted metal is remove.
3M do a weld through Zinc oxide based paint in aerosol form that I have used on my restos in the past although this is more applicable to spot welded panels it does work well of MIG/TIG welding too.
As butt welding on a large panel can lead to distortion most tend to over lap panels.
If butt welded then the problems are largely negated as the reaction tends to occur at the point of the weld joint,and you are better able to seal the the repair so there is less propensity for moisture trapping.
Another important area for potential problems largely ignored is at the filling/ leveling stage, I dont know of many repairers who lead load and metal finish repairs any more, most tend to use a polyester body filler and this will hold water if wet flatting.
If the underlying panel is not treated before filler application then at the the wet flatting/ guide coat stage moisture can be retained in the filler and permeates through to untreated surface beneath promoting early failure - like when you see bubbling around the arc of the wheel arch repair this is normally where the filler feathers into the surrounding panel- if under lying panels are treated it may be trapped in the filler before the final primer coat is applied leading to potential surface finish defects.
Surface finish problems caused by water entrapment are more noticeable if a low bake oven is used to cure the top coat and present as micro blistering in the finish coat or under the lacquer on metallic.
Such is the economics of the modern body repair business that insufficient time is allowed for this moisture to evaporate out of the filler via normal evaporation or a short burst in the oven prior to final priming.
Smaller repairers may use air drying materials and this can hide potential problem or errors made during the preparation.
So in a nutshell proper surface treatments during, after, and prior to painting are the most important factors in the longevity of weld repairs, as well as the obvious sealing of the internals before signing it off.
In the case of Bongo wheel arches, despite some MOT stations using corrosion here as a reason for rejection they are not strictly a structural panel although they could be deemed as a supporting panel if the rusting were extensive or in a prescribed area.
In view of the above, if I were repairing my Bongo arches mainly for cosmetic reasons and they were not to far gone
, I would be looking at dry repair methods.
If done correctly you would be less likely to see premature failure than a poorly welded/ prepped panel repair and you can DIY at a lot less cost too.
I've has some small sections done in the past and whilst they looked good at the time I too have been disappointed by the lack of time before it reappeared. Now its getting more serious and clearly approaching the full replacement stage, which is leaving me wondering about the best course of action.
. gotta keep both eyes on that tin worm.