Rayban,
Maybe if you used a small paintbrush an painted Wyo-Sheen directly on the sewn seam (the thread), the dye wouldn't penetrate the Wyo-Sheen. You could then do any finish you needed to and the thread would stay white. I have not had to use Wyo-Sheen for that, but it seems to me it might just work. Good luck.
Claude Scott
Flying 'S' Leather