The front wings on a mk3/4/5 aren't structural so don't need to be welded on.......(it's the welded edges on the lower part where the rust starts).....so just bolt/screw/rivet them on......this allows you to coat all the mating surfaces before you fit them......(which doesn't get burned off like it does when they are welded on
Use a decent quality etch primer on any bare metal......then paint any 'behind the scenes' and mating surface
areas with a brush (as said above...."you get a thicker coat")
When you do the final fit use a bit of PU adhesive on the mating surfaces to prevent water from being 'sucked' into the joint