Megs SwirlX is more aggressive than Step 1, 3m Scratch remover as JM said is like liquid sandpaper, you could even feel the grit by hand and its definitely very aggressive, do this if you drop by Ace or Handyman take a sachet of Scratch Remover and Heavy Duty Compound, squeeze the sachet and feel both their grits you could say its pretty much the same.
For exteriors 1000 to 1500 grit are advisable but since its mudguard you could use 320 to 500 grit





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