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One tool to avoid while doing most metalwork is a common 4-inch electric grinder with a hard stone. While the grinder isn't the problem, it is somewhat cumbersome compared to svelte air tools, but it can be used with good results if that's all you have. The hard grinding stone is what can potentially make a scarred mess of your work. Hard stones dig into thin sheetmetal and remove lots of material fast, which is bad! If you have a grinder like this, go buy a rubber backing pad to use with 36-grit paper grinding discs. If you use 36-grit paper discs for grinding down welds, the end result will look much nicer.