teak will bring up the "green oil", could be that.. ? Though green oil
is pretty darn obvious, not black at all.
It is strange that the spots get bigger as you sand, that implies a veneer. I might be worth the money
to get a professional varnish
person over to your boat to consult the situation.
Best way to tell if you have fixed the problem is before you lay a coat of varnish
on, saturate wood with denatured alcohol, if the black spots don't appear then more than likely you have fixed the problem. In other words, the way it looks wet is pretty much how it will look varnished.
note: the wood will be a bit lighter with multiple coats of varnish, the varnish creates a prism effect ( I call it prism effect, not an official varnish term I think
). BUT varnish will not cover black spots, no matter how many coats you apply.
Another option is to leave it bare for a while to let the teak recover, it is amazing how teak will repair itself, though again, burnt is burnt, and veneer sand through is not fixable.
And finally, marks and such give teak character, so it doesn't have to be perfect.
Hope that helps,
Mrs. Rain Dog
aka Queen Varnisha