Mig welder question

Hello, I am new to this newsgroup and never done any electric welding until recently. I have a few questions and wondering if anyone could explain and
perhaps give me a few tips.
I have recently bought a Lincoln Electric Mig welder (115 volts unit with 60 amp @ 15amp circuit or 88 amp to 20amp circuit). The unit can be fitted with Argon gas (separate regulator is required) however I choose to go with flux core wirefeed. I am trying to weld car body panel (20 awg sheet metal to existing floor pan of my car).
Questions: Sheet metal preparation: Electric wire brush on clean metal is it okay? Also sheet metal come with like coated oil to prevent rust, can I wipe it down with a rag or do I have to clean it with cleaning solvent?
After about 1 hour on and off welding, my thumb (triggering thumb) feel funny for hours after. Why?
How dangerous is the fume from flux core if inhale?
Is it necessary to run a solid seam of or tack the metal at 1/2 inch distance?
The welder manual said when using flux core, the power must be reversed (which I did) why?
Thx in advance for your help.
Paul
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Wire brushing is always nice and good insurance. With steel, wiping the trace amounts of oil off with a rag is adequate. If your joints are aching possibly you are gripping things too hard. The fumes are not good for you but you are not going to drop dead tomorrow. There is never too much ventilation when using self shielding flux cored wire If you haven't run a solid seam from one corner to another then you are in for a learning experience. Take some scrap and weld a long seam in one pass. Compare it to tacking and back welding. Back welding is when you run your bead back towards your original weld end then jump ahead and repeat. On light gauge a tack every inch should be enough unless you had to force the material into position and it needed the tack to hold it in place. Polarity of your weld current will change the degree of penetration as will the gases ionized at the arc. Your self shielding wire is designed to be run electrode negative unlike bare wire and shielding gas. Randy

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Thanks for the information. I will put on a dusk mask on while welding to block some of the fume. Yeah my welding is not too elegant. Some area like a cold spot that I have to go back to fix. As mentioned. I am new to this and so far I like welding. I am glad that you answer my question about by numb thumb.as I am a bit concern. My thick leather welding glove force me to press the trigger harder. I am going to change that glove to a lighter one. I am planning to do some more welding today.

with
it
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If you run one solid bead instead of back welding you are very likely to get warpage from too much heat.
Jim

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Thanks for the information. 1/ How clean do I have to remove the flux residue to prevent future corrosion before painting over with rust inhibitor paint?
2/ I am still not certain about reverse polarity. As Randy said, reverse electrode polarity increase penetration. How could that happen? Which one has more power. Is the "neg" or "pos" electrode has more penetration power?

explain
go
wipe
feel
reversed
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Hey Paul, since you are using flux core self shielded wire, forget about more or less penetration. The process calls for "electrode negative" otherwise known as "straight polarity".
The only reason for removing the flux is so the paint sticks to the metal. If you paint over the flux it may fall off later leaving the metal unprotected. Hope this helps..

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