ive plastic welded my bumpers, i used a bog standard soldering iron
i smoothed the front bumper, i used blanks cut from a spare rear bumper and i made my own rods from the rear bumper lol (hopefully you have a complete mk4 golf bumper to make your welding rods from)
its pretty simple, just make sure the reccess piece is about 3mm widert than the hole on the mk3 bumper,then you can use that excess plastic to weld and melt into the mk3 bumper, use the rods then to add strength and fill in places.
after that simply smooth the plastic welds, fill prime and paint
ive even had a crash, bumper bent and the filler cracked but my welds stayed strong
so you got no worries with strength
hope this helps
