One of the problems when or rather opportunity to solve something is moisture.
Often in old Victorian houses the air bricks have been blocked up and the void under the house gets damper and damper.
As a floor sander in Bath we often come across this issue.
It is often compounded by the floor boards being overlaid with carpet and underlay. That really stifles the draft !
The moisture often manifests itself as small black rings around the nails. The nails are an expressway for moisture and when wet rust ! staining the floor.
Before buying the floor finish materials you should test the floor With a pin meter and, a high W M E reading (over 14%)
If you observe you see the black spots and the reading is high.
You should investigate further “ make sure air bricks clear”.
You should also consider using a Hard Wax oil as opposed to a water based laquer which might react with the high moisture content.
As a floor sanding company in and around Bath , we would try to identify this problem before we start.