i need suggestion from y'all here.before this,i've tried cleaning my NH-D14 with detergent soap and bucket full of cold water because i wanted to get rid of the crust/staining around the edges of the fins.

i did not dare scrubbing it with any brush because it is thinly nickel coated,scared will lose the coating.

any suggestion on how to get rid of this 'baked' like dust stains on the fins without damaging it?