Weld thru primer?

[jaredts]

I'm about to weld up and repair the passenger side of my cab.  I used aerosol SEM weld thru primer on the driver's side, and I wasn't impressed.  It sprayed like doo-doo and splattered everywhere.  I was thinking of a brush-on this time.  Any opinions?  Do some of you guys avoid using weld-thru primer altogether and why?  I admit there were several plug welds that didn't get this stuff.  I even used Rust-oleum cold galvanizing primer on a couple of spots, but won't know for some years if it works to prevent rust.

[Irish_Alley]

I know at the welding shop I worked at whenever we welded on galv we would use cold galv to cover the welds. and that was on bridges and stuff so I guess it holds up well if we used it for stuff that would see sea salt

[jaredts]

Thanks.  I guess I'm wondering if I'm wasting my time using anything.  Wondering if its good enough just to scratch away ep primer through the plug weld hole and weld away.  I did that on some of my welds on the driver's side.  I know some of the primer gets blistered from the heat and it will rust to some degree.  On the support channel under the floor for instance, there's a layer of surface rust anyway, so I can't imagine that welded spots would rust any faster than everything else.  Who here skips weld thru primer and who believes in using it religiously and why?

[Irish_Alley]

What I was talking about we put the cold galv on after the weld cools even with reg primer I would wait till the weld cools before spaying

[jaredts]

I'm not referring to priming after welding.  Weld thru primer is generally used on the backsides of a surface to be welded.  The zinc flows into the weld so that a surface you cannot get to has some rust protection.  If you just use primer and cannot access between two panels after welding the primer has burned off and will rust around plug welds.  That is the theory behind using it, I was wondering if everyone was sold on that as a necessity.  GM, for example, requires it for their body repairs, but I think Chrysler does not and uses different methods.  Not really sure as I have read some that say not to use it and instead just to scratch off ep coating through the plug weld hole.  Sometimes you are welding to a panel with surface rust on it, though.

[Irish_Alley]

grind the rust away with a wire brush. are you talking about lap weld or a but weld? with a lap weld you finish the weld  and seal the back side with a sealent, on a butt weld well if you primer it after the weld it is all good. i would just clean the metal and weld it then spay primer on it seal the inside if you can