Biggest "technical" mistake I made when I started (and I'm almost too embarrassed to write it) was NOT using a
magnet EVERY time I bought aluminum, copper etc. I'd bought some stuff from this guy who told me he had a roll of copper wire to sell me. I bought it without putting a magnet to it "just this once". I paid $40 for a roll of copper-colored steel wire.
Second - stick with what you KNOW until you learn or gain experience with the other stuff that will come your way (ie: catalytic converters if your not a mechanic and sometimes even then).
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