with a soft metal like that Don't use a sisal it wil tear it up ... you can give this a try :
1. wet sand up to 2500 grit
2. sprial /green color motion
3. loose / white colot motiom
4. loose /red

the black is to coarse you be used on soft metal's.. and as the metal heat's it get's softer than before ... and that is where the fine scratches may be coming from