Weld Thru Primer on backside of butt weld?

[floatinghead]

I'm welding in a patch section on the frame rail on my old BMW and wondering if I should spray the area on the backside ( that will be boxed in once welded) with Weld Thru Primer (WTP)? I will be butt welding, not overlapping this section. I painted the backside with Eastwood rust encapsulator starting 1" away from the weld area, but wasn't sure if I should leave the weld zone bare or WTP on the back? My thought was it wouldn't hurt to be there and may offer better protection. not sure. I plan on spraying a cavity wax once complete, but a bit paranoid it won't be reaching all the right spots.

thanks,

Baron
pacific northwest, USA

 

[HughF]

I assume you're using zintec steel? I'm which case I wouldn't worry about the backside. Cavity wax will take care of it.

If you're not using zintec then you should be.

 

[floatinghead]

thanks for the response. Not using zintec, just mild steel. I guess I'll spray it up and get started on welding it in.

 

[dave h 2]

What Hugh said, what ever they call zinc electro plated steel sheet in America.
Just paint on the back, the burn zone should be fairly narrow.
Lapped plate just causes a moisture trap, had a 76 Trans am with bubbles of rust through at the weld line of all the lapped plates.

 

[HughF]

I actually now do all my repair panels from resin coated roofing steel, it's galvanised, painted and then coated in whatever resin system the manufacturer decided to use.

As I'm usually repairing stuff that isn't seen, it's an ideal material as it comes with the best weather/moisture protection you could probably get.