I wouldn't be using sand on panels, there is probably a good reason why the pros use garnet, wall nuts etc. My guess is if too heavy it could generate too much heat and distort panels.
Pretty sure you don't need to etch after blasting as blasting provides the key. The guys over here diamond bead blast and then apply 2 pac primer.
If you use phosphoric acid with your scotch pads it will be faster and be treating at the same time. Again don't etch an acid treatment. Pics are before and after of a test section I did on an mg guard years ago, just scotch pad and diluted acid. Sometimes it goes a bit yellow, sometimes purple.
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