What welder are you using?220v?110v?..what brand?..Fluxcore or MIG?..Fluxcore is a real PITA to weld any thin sheetmetal even for a well seasoned weldor.
If using Fluxcore and it is your only option,set to the lowest heat possible and pratice on running the wire speed a lil faster than usual until you can JUST spot tack without burning through,clean the slag off after every tack before tacking that area again...do the tacks in complete opposite sides back n forth allowing the tacked areas to cool before continuing in that area until it is completely filled/welded.
If it already ripped in that area and just trying to patch the same area up,you will need to build some kind of structural support there to prevent it from happening again.
If the sheetmetal is has alot of rust behind what you are trying to weld,this will cause "blow throughs" too,rust will easily cause an "arc blow" because the arc is trying to get through the rust,plus the rust will blow out,causing too much heat in the sheetmetal before you actualy get a tack.And rust will cause a very poor weldment in anything.
Pics would be nice to see what you are dealing with too.
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