Scratches that show up white

Fine pale lines catching the light, most visible on darker stains. Run a fingernail across: if it does not catch, the scratch is in the finish only.

What causes it

In rough order of likelihood. Each one has a tell — the observation that separates it from the others.

1. 1

   Scratching in the finish layer only

   How to tell: A fingernail glides over it, and it largely disappears when the floor is damp. This is the good case.

2. 2

   Grit tracked in from outside

   How to tell: Concentrated in traffic paths and near entries. Winter sand and salt are abrasive.

3. 3

   Pet claws

   How to tell: Clusters of parallel lines, worst near doors and at the bottom of the stairs.

4. 4

   The scratch has gone into the wood

   How to tell: A fingernail catches, and it stays visible when damp. Deeper than a recoat will fix.

What it actually means

White is light scattering in a scratch. Whether it is in the finish or in the wood decides whether this costs a few hundred dollars or a few thousand — and the fingernail test answers it in ten seconds.

Finish-only scratching across a whole floor is the textbook case for a screen-and-recoat: one day, no dust of a full sand, and a fraction of the cost. Scratches into the wood need sanding. Isolated deep ones can sometimes be blended rather than doing the whole floor.

• Do the fingernail test in several places. It is the whole diagnosis.
• Mats at every entry, felt pads under everything that moves, and keep dogs' nails trimmed.
• Do not use a scratch-cover or restorer product. It masks the problem for a month and contaminates the floor for the recoat that would actually have fixed it.

• Worn or bare patches on stair treads
• Dull worn paths through the middle of the room