looked like theres a white fluff around the welds, I have only seen that on metal thats been treated. Maybe its just different suppliers but the steel sheets I always get shinny steel.
That looks like what we call Zintec, steel that has been treated with zinc powder rather than hot dip galvanising. It's light grey in colour and will produce the same horrible effects as welding galvanised steel.
This stuff has the same properties as galvanised, so yes, the outer layer will be rustproof except edges that you have cut or areas where you have welded and the coating has burnt off. You'll notice the difference between the coating and the metal on edges you cut or grind, or if you linish the surface you'll go straight through to the bare mild steel (which will be shiny in contrast to the dull grey coating).
As a quick fix rust proofing before you do the final paintwork, buy yourself some cold galvanising spray, should be able to get from Halfords, just spray on welds, the back of welds and fresh edges. It works the same as any other aerosol spray, and dries off pretty much instantly if it's not too thick.
thanks alot for that info m8 ,cause rustproofing is badly needed on these vans as you probably know , its good to know about that sheetmetal too ,now i know what good metal too look for