What has caused these vertical marks?

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Andy's Shed

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Ignore the now very obvious sanding marks, but what are these almost verticle marks in this piece of wood?

I sanded from 80 to 600 with the cup spinning on the lathe at it's slowest speed, then I noticed the light coloured markings and sanded again from 80 back up to 600. I thought the blemishes had gone, but no they're back! What are they?

I'll have to put it back on the lathe anyway and try and remove 'my' sanding scratches, not sure what can be done about those lighter marks though?

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Thanks Stig.
I used a large bowl scraper on the inside, my gouges were catching and I soon gave up using them. I'll have another go with sandpaper.
 
Any kind of scraper is just going to make it worse. I would use a long grind on a spindle or bowl gouge and do a very light shear cut using the side grind and the lathe at high speed. Another way is to use a gouge with a regular finger nail profile but ground at 60-70º Again very light cuts at high speed just using the very tip of the gouge. Coarse sandpaper is just going to pull more fibres, this has to be solved by cutting, make sure your tools are razor sharp, preferably honed and stropped.
 
I wonder whether the tearout may be made worse by your tool 'bouncing' over those little knots. As Oldpete says, higher speed should help, and possibly a bit of support on the outside of the cup to reduce any chatter.
 
That is unusual, I concur with Paul about possible fissures.
That does like like a problematic piece.
 

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