had my parquet flooring sanded back, re-stained and polished by a contractor. But now notice this yucky black residue all around the edge of the floor at the skirting boards. It is rock hard and not eaily removable. Any idea for removing / cleaning / concealing?
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You'd have to remove the skirting boards/baseboards and sand the floor all the way to the edge, which the contractor did not do (and probably based their quote on not doing, as it's a lot more work and risk than just sanding the floor up to the edges without prying anything off the walls.)
That's not "residue;" it's the original surface of the floor before sanding.
Ecnerwal
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Red oak / white oak is loaded with tannins. And also high enough concentration of iron is left in the wood after rapid kiln drying. There are quite a few other sources of iron possible as well- eg flooring nails/staples, etc. Add water and oxygen and you make rust!
Presumably there is water in the water based Poly-urethane finishes and also water based stains.
It helps to understand the internal structure of the wood fibers of oak, which short version is : dense network of small to very small tubules. remember, these trees are very good at moving water all throughout their wood). You can take a length of oak and blow on one end and have air coming out 6' at the other end.
Over time, via moisture, humidity, more-so water spills - even droplets left on the floor or the odd ice cube unseen.. this water will surprisingly efficiently capilary action itself sometimes up to 6 or 8 feet away!! The wood is really thirsty for water, and is being held in an unnaturally dry state against its will via coats of urethane.
Anyway, I offer a slightly different explanation of the source of the black stuff- classic symptom is that it takes a couple days to start showing up more intensely under the finish and generally will continue to darken a bit / show up to other areas, but then as the rust production is limited by Oxygen and Water, it will eventually cease to increase but wont go away on its own.
The only solution I have come across that is effective is to treat the affected areas with concentrated Hydrochloric Acid. Sold as Muriatic acid in hardware stores, everywhere. I don't dilute b/c we dont want to add any more water- that's what were fighting. I apply with q-tips or very small brushes, wear a chem respirator!! and I dont wear gloves but I do wash hands soon after I coat the areas. A little goes a long way. These are by no means the full extent of protective gear you should don- search Google for even more protection ideas. strong ventilation away from you is very good idea, as well as protect or clean any stainless steel appliances around you. (sadly our top of the line stainless vikiing, thermador, and subzero kitchen appliances are technicaally Stainless, but its' the cheap kind that WILL rust if strongly encouraged to do so). Not the end of the world, as the rust will clean off with liquid Stainless steel polish and a little buffing with a soft rag.
Let sit a few minutes or even longer. Most will evaporate and it's not a bad idea to wipe the now dry area with a rag or paper towel that is also dry or blow some air over it to help get rid of any remaining free hydrochlorics lingering around the surface of the wood. DO NOT USE WATER. It may seem awkward using a dry cloth to wipe a dry surface but just run with it. The wood will be very hungry for the acid and laps it right up. You'll notice it doesn't really wet the surface too much.
The wood will turn a beautiful shade of natural wood color. but this fades / mutes within a couple hours to return to the normal color but WITHOUT the black specular or veiny deposits you see in your pictures.
I usually sand moderately with fine grit- eg anything convenienttly available between 120 and 220 grit. Usually by hand as I find random oribitals to be infuriatingly tediously slow to remove any material at all... You dont have to sand much, just a little to return the grain to match neighboring wood. Re-finish with a couple coats of Urethane.
I only use oil based urethanes and even stains. But this technique is compatible witgh the water based ( which you should not be using bc of oaks's tendency to harbor tannins and hence develop black rust as the water based stuff leaves water imbedded in the surface of the wood)
