Dark Marks while sanding

[Kerrowman]

I was sanding the second bowl I had turned using 240grit Aluminium Oxide paper and I noticed dark streaks on the wood (see pic).

Am I right in thinking this is residue from the sand paper that is entering the end grain of the turned bowl?

If so is this normal and should I not use this type of sand paper?

I switched to using some Hope discs on a drill arbor and that got rid of the marks.

Thanks

 

[Duncan A]

It is possibly something from the abrasive, but I think it's far more likely to be torn or bruised wood fibres from poor tool control - too much pressure/wrong angle of presentation. This will improve with practice.
This fibre damage can extend quite deeply into the wood and be a right pig to get rid of but power sanding may get you there quicker as you found!
Duncan

 

[Kerrowman]

Sounds plausible but I’ve never had this effect when sanding items turned end to end (parallel grain) but only when sanding bowls. The finish before sanding seemed good enough with no obvious damage and I’ve followed good advice etc on tool use.

Perhaps I don’t yet recognise such damage? I attach the closest photo of the area I have.