Which weld lap or butt

[zippy500]

I have been looking at various patch panel welding such as floor pans, inner wing and boot area on you tube. I see that professionally body shops are just lap welding panels in (after cutting out the rust), doesn't this a exposed edge on one side ?
I suppose it does the job. I thought that butt welding was the best way to weld a section in.

Just think for when I start cutting rust sections out.

What are peoples view on this

[Alec]

Hello Zippy,

butt weld every time, lap welding leaves a gap around the lap that will attract moisture so encouraging rust. It's quicker to lap weld so presumably why some 'professionals' use that method.
You can of course seam seal a lap weld but still I favour butt welding.

Alec

[bmcecosse]

As above - ideally you cut out all the rust and then cut/form a patch to exactly fill the hole - secure it with magnets and tack it in place. Then weld all around - short bursts and move from side to side to even out the heat and minimise distortion, then dress it down - reweld where required - and job done !

[chickenjohn]

As above, it is far better to accurately cut repair sections and butt weld them in. That was you can paint both sides and there are no hidden overlaps with bare metal present to encourage the start of rust.

[MarkyB]

to accurately cut repair sections

Easier said than done!
Anyone got any good techniques to share?

[zippy500]

to accurately cut repair sections

Easier said than done!
Anyone got any good techniques to share?

I think alot of people use a dremel to make cuts

[Alec]

Hello Marky,

card template marked from the hole, transfer to the steel and my preference is aviation snips for accurate cutting. Angle grinder with a flap disc if some very minor dressing is required.
I see a lot of people mention Dremel but to me it's a toy in the context of car body work.

Alec

[bmcecosse]

Absolutely -Dremel is indeed for toys/models only. Useless on a car!

[Ratbag]

Lap welding is perfectly OK if you can weld both sides of the join.

[chickenjohn]

to accurately cut repair sections

Easier said than done!
Anyone got any good techniques to share?

make a cardboard template to replace all the holed/ rusty metal in the area to be repaired. The make the repair section from this template. When the repair section perfectly conforms to the shape of the area to be replaced, make sure the new section is in the right place and scribe around the repair section. Cut just inside the scribe line and file/ grind till the new piece fits perfectly. A 1mm gap in a few places is OK if your mig welding is good as you will be able to fill this gap with weld.Then grind down the welds.

[polo2k]

another way to get the right size is to place some material behind where you would like to repair and then squirt some cheap paint around the join (I use a zinc primer that contrasts to the panel colour), then cut 1mm outside the line that it makes.

When you have to weld and there is a gap, try backing the area up with a small lump of polished copper or brass. Careful because it will soak up the heat and hold it for a while. If the surface isnt polished then it will stick to the weld. If you dont have copper lying about, you can use flappened 15mm copper, central heating pipe.

[dp]

for making templates, I've found old card document folders (Lever arch type thing) are very easy to do accurately. Ditto old cornflake packets. Cardboard as in corrugated is harder to cut accurately and doesn't curve as nicely.

[les]

I've found leaving a small gap all round the panel to be let in, helps to minimise distortion from the metal expanding. Tacking in place to keep the gap even, then welding short runs in opposite places with (as mentioned) a heat sink behind. However don't expect a perfectly flat result!