Black flecks in was wet now dry surface

[BobL]

I have been building a ww lathe bench and the top is made from a recycled laboratory bench top and I think it is Tassie oak. It had what looks like several coats of partially scratched clear epoxy which I removed pretty easily with a belt sander using an 80 grit belt. I then sanded it with 120 and then 240 grit so it looked and felt pretty smooth at this stage (remember it only has to be a workbench). I then did my usual method of raising the grain by spraying a fine mist of water on it whereby it developed lots of little black spots and grey patches that now cannot be easily sanded out. I have seen this before on spotted gum but not on Tassie oak, not that I have worked a lot with TO. Does anyone have any ideas what causes this and how to avoid it.

I thought it might have something to do with it being a lab bench but it was a dry lab (i.e. no chemicals) and the black/grey does not appear to coincide with where the scratches in the epoxy where at their densest.

[rwbuild]

http://constructionchemicals.co.uk/acatalog/Oxalic10.pdf

Good info here using Oxalic acid. Should be able to get it from a specialist paint store or trade paint specialist

[BobL]

OK I think I know what might have happened. I was doing the sanding near where my metal work grinders are located and after starting the sanding I stopped and did a metal work job. This sprayed metal dust onto the bench . Then I did the final sand of the bench top and metal dust was embedded into the bench top and this turned black when I sprayed the water on it. If the oxalic acid works this will confirm this.

[rwbuild]

It is a powder or flakes and it is mixed with hot water. I have a house painter who uses it to get metal rust out of maple and western red cedar so it can be stained instead of painting. Works in 98% of the time, the other 2%, he just darkens the stain a tad so you don't noticeably see it unless you were told it was there. I'm confident it will work for you. Can also use ammonia bleach afterwards if there is still a hint of color but fade it out over a wider area so it is not as noticeable. :ok:

[BobL]

I have managed to get some oxalic acid from work and will give it a go this evening. I read up the MSDS so I can handle it accordingly.

[BobL]

Well that Oxalic Acid sure did the trick.
Before - see all the grey black flecks and streaks
http://www.woodworkforums.com/attachment.php?attachmentid=214335&stc=1&d=1341225469
http://www.woodworkforums.com/attachment.php?attachmentid=214336&stc=1&d=1341225474

After 1 minute of 10% oxalic acid.
http://www.woodworkforums.com/attachment.php?attachmentid=214338&stc=1&d=1341225474
http://www.woodworkforums.com/attachment.php?attachmentid=214337&stc=1&d=1341225474