welding in rust

I read the information in Tech Talk comparing Por-15 Rust Bullet and Epoxy mastic 121. I have been using Por-15 so I bought a litre of Rust Bullet and 2.5 litres of Epoxy mastic 121, to try. If someone can help, my question is when I remove rust e.g. at the bottom of the engine lid where they all rust, I have drilled the spot welds out to remove the inner panel, cut the rust out, went through the Rust Bullet process now what happens when I weld the panel back in? All the rust prevention will be burnt off. How do you prevent this?

Rust bullet does not take weld too good. Buff back just what you need to then coat with weld through primer (copper rich).
Then get what you after with rust bullet. Follow it with penatrol to vet the last of it.

There are numerous 'weld through primers' The Copper primers spatter less when MIG welded. Any Automotive paint supply place will have it. Dont use Cold Galvanising Primer. It will spatter huge and blow out the weld.

I've used galvanised primer without any spatter issue for plug welds. Some say a slightly higher setting helps get thru the primer but if it's good quality, like Wurth, then should be fine.

Once welded n cleaned up I thin the rust paint of choice so that it can flow into the join between the sheets of metal. Then for overkill some rustbuster wax or Waxoyl, cos it's got inhibitors in it, or Bilt and Hamber ( tho the Oz importer WAY over charges for it but your choice )

Thank you for your help. I can see the point behind putting wax etc. on after welding. However wouldn't the weld through primer burn off as well and welds rust so quickly. what do they do in the factory?

gt_51 wrote:what do they do in the factory?

They dip it.

The factory spot welds the panels with robots now. Then does an electrocoat dip.. The Copper weld primer is the best chance you have.

Steel is iron. If it is iron I assure it will rust. Cannot stop it.

Thank you for all that info, I will get some copper weld primer, then after I have welded the inner panel I will try and get some more Rust Bullet in there.

The one that I have had success with is U-Pol Weld Thru No2
It is in a silver tall can with green writing from memory. U Pol as a 1-4 numbered range with each number a different thing.

68BUS wrote:The one that I have had success with is U-Pol Weld Thru No2
It is in a silver tall can with green writing from memory. U Pol as a 1-4 numbered range with each number a different thing.

Yup, this stuff is good too. I thunk #2 is zinc and #3 is copper. Can't quite remember.

as for burning off. Yes it does on the weld and a bit around it but the rest is fine and hence gives protection. As I said, thinned down rust inhibiting paint will fill the gap between the panels and so any moisture caught near the weld will take a bloody long time to rust out. if you use a moisture curing paint, like KBS, then I believe that won't be an issue.