Basement entry door maintenance

My steel basement entry doors are now about 13 years old and showing signs of rust spots. Mostly at corners and spots where ice icicles may have fallen. No perforation noted yet.
Intent is to grind down and prep for painting while the weather is still good. Any recommendations on products such as primers best used for spot treatment? At this time I don't intend on prepping the entire door assembly for repainting.
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