I have a Harbor Freight flux core 90 amp welder--it "works", but it is NOT a "MIG"..it is a wire fed ARC welder!..people think because a welder uses wire feed on a spool ,its a MIG,but nope--not if it is flux core,its simply an arc welder that has flux coated wire instead of using welding rods..a true MIG uses argon & CO2 gas and bare wire..
I haven't tried using mine much,but long enough to see it sucks for doing sheet metal,I can tack weld thin stuff better with my AC buzz box using 6013 rods than the wire feed flux core,even with the flux core on the "low" setting,it still blows holes too easily..(it only has "hi amd lo",no other heat settings..
I tried using it once to weld nuts to bolts I had to chop the heads off on exhaust manifolds and though it appeared to make a nice solid looking bead,the weld was so brittle it snapped like glass when I tried turning the nut with a wrench with hardly any effort..I ended up brazing the nuts on with the torch,and that did work..
I'm glad I only gave $30 for the welder--the cart it came with was worth $50 to me,the welder I figured was not so great,and I considered it a bonus--it still had the original spool of wire in it--the guy I bought it from wasn't highly impressed with it either I assume..looked like it was used for 10 minutes!..he bought a 220V true MIG to finish his '48 Pontiac street rod project.
Maybe if I practiced with it more I'd be able to get better results,but the wire feed on it seems to skip and sounds like the motor is having a hernia,so maybe not..this thing would be good for welding 1/8" to 3/16" steel,but its too hot to do thinner metal,and not hot enough to weld 1/4" well..
I would say get a REAL MIG welder,one that runs on 220V and uses gas..then you'll be able to weld anything on a truck,from sheet metal all the way up to 3/8" thick like axle tubes,trailer hitches and frames..the 110V ones are OK for most jobs but sometimes you'll want the ability to mate two thick pieces in one pass..the 110V ones do have one advantage,no special wiring or outlet is needed,just a 20 to 30 amp circuit..