How to make sanded pine floor boards look old again

[Badger35]

I have a 1980s house and when we moved in I sanded all the upstairs pine floor boards with a water based varnish. After five years the bathroom needs doing again and I have some varnish left. However, I have had to sand back quite a bit to remove the old varnish and because the boards are slightly 'cupped' there are now some lighter strips at the edges of the boards. Is there anything I can do please?

I have seen stuff online suggesting vinegar and wire wool, baking powder, tea bags, and potassium permanganate. I have also read somewhere that the bare pine will darken again with age any way, but will this happen under vanish, or would it be better to live with bare boards for a while? How long would it take?

 

[piglet11]

Floorboards in a 1980's house is a bit unusual, chipboard being the norm then.
I guess the problem is that natural timber in a moist atmosphere is going to move, and depending on how the timber was cut at the sawmill, it might have that tendency to curl.
The other factor is you are varnishing one side and leaving the underside exposed - this can further cause warping. It's the reason veneered timber will have a balancing veneer on the other side to counteract the tendency to warp.
Sanding off the high spots reduces the thickness and might accelerate the process.
The other problem is that boards put in by builders are really only intended as sub-floors, not as the final surface. If nailed, the nails can pull through with movement over time
Engineered boards are a different kettle of fish and are designed not to warp and provide a final finished floor.
Pine will yellow with age but could take a long time.
I can't advise on the remedies you found. Tea bags might act as a stain and Potassium Permanganate certainly will, provided you like purple floorboards.
Another problem with something like tongue and groove boarding is that it leaks like a sieve and normally there a lot of air movement between the floor and ceiling below. This can lead to cold draughts and staining of carpets.
I wonder about the wisdom of what I guess is more of a structural timber being used in a bathroom at all and that tiling might be a better long term option.

 

[CACJL]

You need a solvent based flooring grade varnish, try Barrettine Coatings and Finishes Polyurethane Gloss Varnish, I found it gives a very good hard finish. Available from Toolstation.
I always give the wood a light sand first then wipe with a White Spirit soaked rag to removed any contamination and it opens up the grain in bare wood.