Black marks from nails on Redwood

[suds]

I'm sanding down some Redwood cap boards for my deck railing and from the nails used previously there atr black stains that don't seem to be able to sand out. The boards are 1/2" Redwood and I'm worried about going too deep. Any hints on how to get rid of the black stains?

 

[sheeschen]

I used oxalic acid to get some metal stains out from some redwood posts on my arbor.  It worked well for surface stains, but I don't know how it would work for more embedded stains.  Worth a shot.  I think I ended up buying it online after coming up empty-handed at some of the local hardware stores.  If you have a chemical supply place nearby, you may be able to get it there.

Here's a source, with some description on using it - http://www.realmilkpaint.com/oxalic-acid-bleach.html