I'm not an expert welder by any means but I still think that ideally stainless filler is called for. From what I've gathered working around professional welders and from reading a little I'd still use a stainless filler in this case.

Not saying a mild steel filler won't work, it should/will, but if going "by the book" it's probably not correct.

Here's some reading about joining dissimilar metals.
http://www.esabna.com/euweb/awtc/Lesson5_18.htm

Welding mild steel to 430 stainless steel.
http://www.mig-welding.co.uk/stainless.htm
If you look in the chart they recommend a 309L filler, which oddly enough is what I guessed (I'm just not a smart enough welder to know why..)

Grigg

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