Preventing rust in an overlap weld joint

[MTBob]

When fabricating an assembly with two pieces of metal overlapping each other, what is the proper method of connecting them to reduce or eliminate rusting in the joint. To be specific, if I have two sections of, say 1/8"x6"x6" steel plate that overlap each other by 3"- and, say I want to weld the two parallel pieces together. And, let's say I want to stitch weld the seams, that is, not a continuous weld for the full length of the overlap. What steps do I take to make sure that, after painting the assembly, that I don't get rust coming out of the area of metal between the two plates. Should I paint the steel before welding(and wire brush off the area to be welded? Any insight would be appreciated.

[deafman]

spray some cold galvanizing on the area to be concealed

The paint dries quickly. Use a flap disk or wire brush to get rid of any spray in the weld zone if you want. Some of it is ruined by the welding heat, but at least you have protection in between the two sheets.

[nfinch86]

When fabricating an assembly with two pieces of metal overlapping each other, what is the proper method of connecting them to reduce or eliminate rusting in the joint. To be specific, if I have two sections of, say 1/8"x6"x6" steel plate that overlap each other by 3"- and, say I want to weld the two parallel pieces together. And, let's say I want to stitch weld the seams, that is, not a continuous weld for the full length of the overlap. What steps do I take to make sure that, after painting the assembly, that I don't get rust coming out of the area of metal between the two plates. Should I paint the steel before welding(and wire brush off the area to be welded? Any insight would be appreciated.

Bob, what are you making ?
I like Deafman's idea, also after it's painted you could silicone in between the welds.

.......... Norm

[sailor]

Immediatly after your welds are cleaned, epoxy paint generously along both edges of your lap weld. The epoxy is a 100% vapor barrier.

[kcstott]

Lets keep in mind that anything applied to low alloy steel is just a delay on the rust setting in.
Even epoxy paint will break down over time and let in moisture.

Lets also keep this relative too. Epoxy paint is just about the best way to easily protect a surface from rust. It will take years for it to degrade but it will still degrade. And that is what needs to be addressed. You will never stop rust. You can only maintain the part/assembly. Depending on the application a new pain job every few years is the best thing, along with realizing that one paint job is not enough.

[Daniel]

like Norm says, put some caulking in between your stitches.

[ShockHazard]

If you can, get your part electroplated PRIOR to painting, that would be the best. A good cadmium (toxic) or zinc (non toxic) with a yellow chromate converstion coating (top coat, also known as "Type II yellow" in the industry) should suit you for a long long time.

I plate all of my parts that I weld PRIOR to painting. It just prolongs the rust problem that much longer.

[nocheepgas]

Use a weld through primer prior to welding. Generally available at autobody stores.

[t1113rs]

Rhino-liner

In farming, they make these big gravity boxes for wagons. The boxes get lowered onto an angle iron frame that is built off of an under-carriage that used to be a big rig before hitting the junk yard. One guy sprayed Rhino-liner all around where the angle-iron contacts the sheet metal and 2 inches out on every side. I thought that was a good idea. Every springtime it would sweat between the sheet metal and the angle-iron and rust would spread out after a few years; and it would never be practical to weld solid on angle-iron to sheet metal. Also, in the spots where the angle iron connects to the under-carriage, this guy welded on fish plates to the frame where he intended to build off of... and then surrounded the whole joint with Rhino-liner.

[t1113rs]

Also, the auto companies used to have all those rust problems that arose out of overlapped joints that had been spot-welded or otherwise overlapped and had been painted over. Their solution was to run the line down into a submersion dunk tank of electroplated paint after the body was completed in the "Body-In-White" stage. Now, all components (even the frame...if applicable) is submerged in paint. Initially, they still neglected the frame. The bought under-carriage paint in bulk and got the cheapest stuff they could. All they cared about was to get the frame "black" because the customer wouldn't see it anyway.

I guess a guy with a weld shop could maintain a sealed tank of highly thinned rustoleum paint in which to submerge parts. Just add more paint and thinner every oncle in a while. It would have to be the size of a parts washer, have an aggitator pump of some kind, and have a gasketed lid so the thinner wouldn't evaporate away.

[seattle smitty]

Don't spray silicone into the joint if there's any possibility that you will want to paint it later . . . .

[piniongear]

What I have used a lot is a primer called 'Weld Thru Primer.'
It is loaded with zinc and you paint the surface of both sides of the plate joint then overlap the plates and weld it up. The primer dries quickly.
As the name implies, you can weld through this material and it protects the steel from then on.
pg

[nfinch86]

Don't spray silicone into the joint if there's any possibility that you will want to paint it later . . . .

As far as I know you can buy Paintable Silicone sealant .

............ Norm

[jbmprods]

As far as I know you can buy Paintable Silicone sealant .

............ Norm

if you are painting it with water based house paint it will work....but if you are using oil or mineral based auto or equipment enamel it will cause fish eyes in the paint...... just a little story from when i was doing body work on cars and trucks. another thing you want to stay far away from when you are painting is wd40. as i was shooting a car open air a guy 50 yrs away sprayed a rusty nut with wd40 and the slight breeze drifted it over to where i was working and presto my job was ruined. fish eyes started popping up everywhere.

[shovelon]

What I have used a lot is a primer called 'Weld Thru Primer.'
It is loaded with zinc and you paint the surface of both sides of the plate joint then overlap the plates and weld it up. The primer dries quickly.
As the name implies, you can weld through this material and it protects the steel from then on.
pg

Excellent!

That is a revelation.

Thanks