tips for mig welding stainless steel?

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hello was wondering if anyone had any advice for mig welding stainless steel.  Ive done a fair amount of mild steel mig welding but am trying out stainless for the first time. I got a bunch of scrap 304 stainless in gauges ranging from 18 to 10 with everything in between.  Ive been running beads on my scrap and the beads look good but it makes a good amount of black/brown soot and the beads themselves are a dark grey.  When i set my parameters on the machine im pretty much using the suggested settings that are on the inside of the welder and than adjusting a little bit as i go to see what that does.  I can clean all the soot off with a ss wire brush and most of the grey color off of the bead but am not sure if the soot and discoloration of the bead is normal for the ss mig process?    Im working with a millermatic 211 230v using .035 308L wire (also have a .030 308lsi wire that I haven't used yet) and the regular gun that comes with the welder and trimix as my shielding gas at 25cfh.  Also have a spool gun but don't know if its needed for ss.  have been wire wheeling my metal with a 304 stainless wire wheel as well.Ultimately ill be welding some 304 stainless round tubing which I think to be around 10 gauge (guessing on the gauge because the tubing tapers from a thinner gauge to a thicker gauge which I will be welding but cant measure) onto some kegs that have 16 gauge walls for the purpose of brewing beer.    when I do my weld on the kegs I will be back gassing but haven't as of yet because I am still figuring out this stainless stuff and didn't want to waste any trimix seeing as I already had to give up my first born and a bunch of dollars to get the gas. so  my questions are is the soot and initial discoloration of the bead normal for the ss mig welding process?  should I preheat the 10 gauge tubing when welding it to the 16 gauge ss?  ive seen some paste you can use instead of back gassing, is that a suitable substitute when mig welding ss?  if anyone has any tip or advice for the ss mig welding im all ears! thanks in advanceLast edited by dogvila; 09-27-2014 at 11:37 PM.
Reply:almost forgot, does anyone have any experience with 308l vs 308lsi?
Reply:not sure on the 308l vs 308lsi question, but here is what I do knowshort-circuit stainless is HARD! at least compared to mild steel. the tad bit I have done has given a good gold to blue color, with tri-mix gas. I want to say a little black soot around the weld was normal. sounds like your running hot unless you are doing.. . . .spray arc stainless. a bit weird, but not too difficult. I was never able to spray anything that didn't come out grey. we did it with 98/2 gas and it was go go go! I doubt your spraying on 1/8" and less, but figured I would throw my experience in it just for reference.you do not need a spoolgunnot trying to bash, and trust me I know cash limits like no other, but it sounds like you need a tig machine to do your project. to get full-pen and sanitary with a wire-feeder on ss is tough.I know this post hasn't been super helpful, but hopefully the "bump" will give a chance for someone with more knowledge to chime inbosses stuff:trailblazer 325maxstar 200my stuff:sa 200fronius transpocket 180100 amp Lincoln w/f97 f350 DITKevin
Reply:dogvilaRefine your question - to a question.Opus
Reply:thanks for the reply 92dlxman, yea I would def prefer to do this project with a tig but just upgraded to the millermatic last week so i don't think the old lady would like me buying another machine haha!  oh well, suppose ill just make do with what I got see how it turns out.  I was mostly worried about the soot I was getting but some more internet searching and your response puts my mind a little more at ease.opus thank you as well for taking the time to respond, very helpful
Reply:For a a few weeks at my old job all I did for my 5hr day was weld 16ga 316 stainless with .023 308l with on a millermatic135. Its not much different then regular wire feed other the sound. After the hoods we were building were finished boss man put a thing called pickling acid in it and it takes the soot and grey off the weld.
Reply:Originally Posted by dogvilaalmost forgot, does anyone have any experience with 308l vs 308lsi?
Reply:thanks for the tip on the pickling swampwelder, hadn't heard of that but after some reading definitely seems to be appropriate for my application.  Will have to read up on it some morethanks for the reply john, ill give that 308lsi a try, I have a spare keg so im gonna give it a shot at some point, if I suck it up than ill take it to some one else to weld, but I certainly wont be happy about itLast edited by dogvila; 09-28-2014 at 10:14 PM.
Reply:hey swampwelder, when you did that thin gauge stuff did you happen to put a copper backing plate behind the metal where you were welding?  I have read that some people do it to help with heat dissipation and also to help for shielding from atmosphere on the back of the weld
Reply:Originally Posted by dogvilahey swampwelder, when you did that thin gauge stuff did you happen to put a copper backing plate behind the metal where you were welding?  I have read that some people do it to help with heat dissipation and also to help for shielding from atmosphere on the back of the weld
Reply:Truly this needs to be done with a TIG process I don't see it being good enough mig where you won't have areas where bacteria could grow.Now for non sanitary stuff what you have is perfect, I use the exact same setup at work. You need the trimix and as long as you stay under 24v your fine, after 24 that machine will go into spray transfer mode.Last edited by Rick_H; 10-02-2014 at 01:04 AM.AWS 17.1, D1.1 and ASME IX (GTAW)Miller Syncrowave 350LX, Maxstar 150sth, Maxstar 200DX and  Millermatic 252 w/ Spoolmatic 30aMiller buzz box with Lincolin TombstoneThermal Dynamics Cutmaster 38