TBH, it's not going to get that bad.
I have had a sheet of 1mm Mild sitting out of the back of the garage in all weather for 18 months. I needed a bit for a repair on the golf. it had surface rust all over. But 2 mins with a sandig disk and the bit i needed was bright and shiney.
I will build mine, etch it and then primer and top coat.
TT
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