Hello guys, my post here. I'm not quite a n00b, having done lots of rough outdoor carpentry, but I'm a novice at finishing. Here's my problem. I need to provide full background.
My wife's house of 35 years was built strangely and has required me to do all kinds of odd maintenance tasks. One of these was the replacement of the front door sill, an unusually thick, wide slab of wood with fiddly cut-outs at the ends. I couldn't find any suitable timber at the local builders depot so went to a timber salvage yard, where I found a piece of white oak large enough to yield both the sill and a much smaller weatherstrip. Shaping it in my small workshop was a challenge. I was generally pleased with the results, but noticed a discoloured patch which I was mostly able to cut away while cutting the slope into the upper surface.
My wife, who did the varnishing, noted that the wood where I'd cut away the discoloration didn't take varnish very well. She used generic "yacht varnish", nothing special, and kept on applying new coats until it looked right.
After a winter, the sill no longer looks right. The hard-to-varnish area on the main sill has acquired a black-to-grey discoloration, and there's a similar, smaller patch on the weatherstrip. I assumed that this was fungal growth because the varnish hadn't bonded properly to the wood, so got some Epifanes varnish, read up on application techniques and began a refinish. Sanding went fine, yielding clean white wood. However, when I washed with acetone, the grey-black colour we'd seen over the winter re-emerged. I'm now thinking that the discoloration isn't from fungus growing on the surface of the wood but is somehow in the grain.
Is this a well-known phenom? Obviously we can paint, but I'd rather avoid the problem by prepping properly.