hey guys just wondering why when i see alot of rust repairs done why people dont weld in stainless sheetmetal instead of mild steel? would last alot longer

You can weld mild steel and stainless together but I believe the two different metals react with each other and cause corrosion between the join. Same if you say put a stainless washer against raw steel.
You could make the whole car out of stainless and avoid the problem!
Stainless is also heavier as it is denser than mild steel.
I'd like to see a fully polished stainless Datto, very nice!

im not to sure about that one mate i know what you mean but if its primed properly why wouldnt it last longer if your only replacing little square sections at a time

The weld zone will be chrome depleated and as shoota said the dissimilar metals is an issue. No worries if it's painted but once the two metals are exposed you have a galvanic cell and accelerated corrosion. It's to do with the galvanic series of metals more reactive metals essentially sacrifice themselves for the less reactive (sacrificial anode). The same reason alloy heads corrode against the block.

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Yes, the stainless eats away the mild steel it is attached to, there was an old steel tank at work which the stainless steel patches fell off, the welds were still intact and the wall thinckness was fine everywhere except for the 2 inches surrounding where the patches were sitting.

it doesnt matter wat steel u use any repair will rust eather side of weld...

Stainless will corrode the surrounding mild steel quicker than any other metal, painted or not.

apart from the galvanic effect. isnt it just kinda pointless to weld in stainless with the aim of having the patch last longer? no point having a completely rusted out datsun with a couple of mint square patches in it

sure if you were somehow making a whole panel out of it there might be some point
the galvanic effect isnt an issue if both sides of the metal are painted...as then there should be corrosion of either metal. just like a rust repair should rust again if it is properly painted, which is usually the issue with patches that you cant get to the back of to paint

a deloreon with a flux compacitor.....no rust problems at all....hah.

it's difficult enough trying to work zinc anneal over cold rolled... stainless is way too hard and way too expensive.