I sanded back to remove the Woodskin, cleaned with white spirit and started applying the Teak Oil until it stopped soaking in. All the washboards took the about 1/3 of a litre of teak oil over a week, a coat brushed on every day, initially twice a day. Once the wood stopped absorbing the teak oil, I wiped off the excess.
They actually look more orange now in the pictures but the photos were taken in different lighting conditions and they look more teak coloured in the real. The sanded picture is a strip light, at night with flash, the oiled picture, todays late afternoon sun. The washboards are a bit of a loose fit, so its their last hurrah. If the teak oil doesn't work, or becomes too much hassle to maintain, I will likely replace them with a clear plastic washboard but in two sections.
The darker strip to the left of the centre line, is caused by a flat security bar bolted across the boards and shading them from the sun.

