flux core welding question

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Hi, I am new here and new to mig welding so bear with me. I recently purchased a small 90 amp flux core welder from harbor freight so I could weld in some floor pans in my Mustang and so I could work with other thin steel to later build a go kart. It will not be used for anything thick so I have no need for a bigger welder. Yes I know gas is better but other than these two projects I have absolutely no use for a welder so that is why I settled for such a small welder without gas. To my surprise this little welder works better than I thought. The spatter isn't too bad if the weld area is good and clean, but if it's not then I get quite a mess from the spatter. But it isn't anything I can't clean up with a grinder or sander.My problem is I am working with thin steel sheet metal between 18-22 gauge. Right now I am practising on sheet metal from my local hardware store until I get this down right. I am attempting to do plug welding and practising butt welding two pieces together. Well since the sheet metal is so thin, I can't weld for very long without burning holes in the sheet metal. Right now I have the welder on its lowest amperage which is about 55 amps I guess since high is 90 and wire speed on like 1 since it seems to do the plug welds best at the lower setting and doesn't pop like it does on the higher wire speeds. Anything faster just melts straight through. My problem still is that even on the lowest setting there is the possibility of burning holes. I have tried a copper backing plate which works great but there will be times when I can't get a copper backing plate behind the floor pans when I am doing them. So I need to try and get a technique down so I don't have to use one. If I don't weld for too long I can get a puddle just level with the top sheet of steel and good penetration showing on the underside piece without it burning through. But I was expecting a higher puddle rising above the top sheet and I would have to grind it down later. Now if I weld for a couple seconds until the puddle is just level with the top sheet and stop to let it cool down, I can then start welding again on top of that to raise the bead higher above the top sheet. That is where my question comes in. Is it ok to start welding for a plug weld, stop so it can cool, then weld again over that? Is there anything I should to ensure it is a strong weld? For instance, when I stop to let the weld cool down, should I use a wire brush to scrape the weld before starting to weld over it again?When I do the butt welding with a copper plate behind the two connecting sheets of steel I can usually weld it better. I basically stitch the sheets together in random spots and then go back and lay a bead along the gap and the copper backing really helps. I can do this and it looks fine, but the plug welds usually do not look so nice. Because of the fact I can't always get copper behind them, such as when I weld my floor pans to the rocker panel, I will have no choice but to go without the backing plate. I just want to make sure it is ok to lay a weld over a weld basically. Like I said, the first weld will give good penetration and a puddle as high as the top sheet of steel, but not always perfectly even. The only way I have found to fix this issue is to weld over the plug again. Then I can grind off the excess to make it look nice and even. Sorry for the long read, but I was hoping someone would have the answer to my question and maybe could offer any other tips. I am just practising with it right now before I touch my cars floor pans. My past experience with welding is limited to arc welding thicker steel. I am not used to working with such thin metal, but I have managed to get a couple strong welds with this little flux welder so I'm sure it can be done. I just need to get a few tips from the experienced welders.Thanks
Reply:You should only go for a short time anyway. The heat will build up real quick and burn through. Weld an inch, jump to another area and weld an inch and so on. It sounds like a pain but it has a few advantages, it won't burn through and look like crap and will also lessen the chance of warping the sheet metal.
Reply:short beads will be the best to be honest for doing sheetmetal work you would have better luck if you could use a gas mig with FC all you can hope for is lots of short beads or track welds  the backing was a good choice.. using the smallest wire you can get will help alsoidealarc 250/250 ac-dc tigidealarc 250/250 ac-dc tig #2 used for sticklincoln sp100hh125dual arbor grinder polisher30 yrs of hand tools52 pitch blocks 6p-26prake gauge -pitch gaugeG&D prop repair 918-207-6938Hulbert,okla 74441
Reply:Originally Posted by prop-doctorshort beads will be the best to be honest for doing sheetmetal work you would have better luck if you could use a gas mig with FC all you can hope for is lots of short beads or track welds  the backing was a good choice.. using the smallest wire you can get will help also
Reply:With FC wire make sure to clean the flux off between welds as you would with stick welding. If you are welding over a previous weld the flux of the first weld needs to be removed before starting the second. Otherwise the flux from the first contaminates the second.
Reply:Wrong process for the material and joint type. Oxy/Fuel, GMAW, GTAW, ERW, would all be much better processes for you.
Reply:I don't do much sheet metal work and when I do I use tig.  So take my advise for what it's worth.Most times when I see guys doing automotive patch panels they just put in a series of tack welds a few inches apart, then fill back in with more tack welds.  They get the tack welds fairly dense (1/4" to 1/2" maybe) before they attempt holding a puddle and running a bead.  Having the two panels connected in many places gives more paths for the heat to equalize across the pieces and helps from eating into one side or the other.Dynasty200DX w/coolmate1MM210MM VintageESAB miniarc161ltsLincoln AC225Victor O/A, Smith AW1ACutmaster 81IR 2475N7.5FPRage3Jancy USA1019" SBAEAD-200LE