when cleaning the surface of say car bodywork, I know it is important to get a shiny surface for welding, but what about the underside. If I am welding into a cavity and can't cleanup the underside will it spoil the weld penetration at all ??
Sometimes you'll get crap bubble to the surface, sometimes the whole lot will spit you in the face and other times you'll set fire to the rustproofing.
Welding cars is lots of fun............
Stop worrying, it's a fact of life.
I've done loads of welding on my car thats been Zeibarted from new, then Waxoiled 20 odd years ago, and it’s a right pain in the jacksy. In a perfect world both sides should be clean, shiny and cleaned off with thinners just for good measure. When you get to work like this its a dream, however in the real world there are loads of occasions when thats impossible.
Get the side you can see nice and clean and get welding. Yes it will smoke and yes the sh1t on the back will come through as a hole now and again, but you can re do that later. Once the offending stuff has burnt of a plenty you can go back over the bad bits.
As ever though make sure you try your settings on a clean bit of similar thickness steel first to make sure you're getting the right penetration.