Many newbie welders make a classic mistake and this is a bad earth connection to their job and this is very common among newbies welding corroded metals often found on cars and other vehicles.
My first bit of advice would be to get a good earth clamp, a good earth clamp IS NOT one of the oversized crocodile clips which come with cheaper welders as usually one pointed tooth is making contact and earthing the full current and begins glowing red hot. A good earth clamp is one with a braided copper strap across both sides of the earth clamp with copper contact pieces on both sides of the clamp, it is NOT reliant on the pivot pin to carry the current to both sides of the clamp, and it needs a high spring pressure to make a good contact with the job.
Next you need clean metal to use the full potential of any earth clamp and for many thicker sections a simple angle grinder is sufficient to remove any paint, rust, or even mill scale from cold rolled material, for thinner metals such as sheet metal uses on bodywork a backing pad and resin bonded discs are better. They are much finer and remove paint and scale easily without the aggressive removal of material and work much better on rusty material. You can even use a stainless steel wire brush on an angle grinder and these are effective in removing loose and fine rust and giving a clean surface for your new earth clamp.
What if you are welding something like a vehicle and cannot get an earth clamp near your weld? take a piece of thicker sheet metal (i prefer 3mm) and cut a strip of it and clean it fully, attach your earth clamp and clean the bodywork near your weld and tack it to your job. Always put 3 good tacks on one side only and this should give sufficient earthing for most jobs, and once you have finished you can often bend it off or cut it off and grind your tacks off with a resin bonded disc.
My first bit of advice would be to get a good earth clamp, a good earth clamp IS NOT one of the oversized crocodile clips which come with cheaper welders as usually one pointed tooth is making contact and earthing the full current and begins glowing red hot. A good earth clamp is one with a braided copper strap across both sides of the earth clamp with copper contact pieces on both sides of the clamp, it is NOT reliant on the pivot pin to carry the current to both sides of the clamp, and it needs a high spring pressure to make a good contact with the job.
Next you need clean metal to use the full potential of any earth clamp and for many thicker sections a simple angle grinder is sufficient to remove any paint, rust, or even mill scale from cold rolled material, for thinner metals such as sheet metal uses on bodywork a backing pad and resin bonded discs are better. They are much finer and remove paint and scale easily without the aggressive removal of material and work much better on rusty material. You can even use a stainless steel wire brush on an angle grinder and these are effective in removing loose and fine rust and giving a clean surface for your new earth clamp.
What if you are welding something like a vehicle and cannot get an earth clamp near your weld? take a piece of thicker sheet metal (i prefer 3mm) and cut a strip of it and clean it fully, attach your earth clamp and clean the bodywork near your weld and tack it to your job. Always put 3 good tacks on one side only and this should give sufficient earthing for most jobs, and once you have finished you can often bend it off or cut it off and grind your tacks off with a resin bonded disc.