I really don't know if there's anything to remove wood staining that went deep inside a very porous stone. It's some tenacious stuff, alright!
The good news is that rain and good old sun are very powerful stain-removers. Give them enough time and they'll take care of it (hopefully in much less than one year…).
As for the sealing, it's never a good idea to apply a sealer for stone to outdoor installations, not even my super-duper excellent MB-4!
May I ask you now to please read and e-sign our Statement of Purpose at: http://www.marblecleaning.org/purpose.htm?
Ciao and good luck,
Article ID: 1592
Created On: Mon, Jul 28, 2008 at 4:01 PM
Last Updated On: Mon, Jul 28, 2008 at 12:00 AM
Authored by: Maurizio Bertoli [firstname.lastname@example.org]
Online URL: https://marblecleaning.org/knowledgebase/article.php?id=1592