How to remove water staining on wooden window cill/board?

[okeydokey]

Some years ago when some double glazing was fitted I had internal wooden window boards fitted, a semi hardwood not a cedar or skirting board type of timber which then had a coat or two (I suspect) of whatever varnish was available maybe yacht varnish but now most of its long gone/worn away.
Over the years the South facing window boards have suffered the stress of UV sunlight light, plants and sundry items with the result that several are in a cosmetically bad state. See photos showing staining.
Ive tried gently sanding but the discolouration is too deep, its been suggested that Oxalic acid or Liberon Ring Remover or Liberon wood bleach would clear them so I can refinish --- any ideas or whether those would work? - and then the next question to finish the surface with a - water based or solvent "varnish" ?
Thanks for your consideration

 

[baldkev]

Hi, make sure the whole surface of the board is free from the previous finish, then mix oxalic with warm / hot water, brush on over the whole cill evenly...... wait a few minutes and then wipe it off with clean water. If this hasnt given the desired effect, go again.

Oxalic is great for this type of issue , but make sure you neutralise it with clean water. Any subsequent sanding should be done with a decent mask as the oxalic crystals can be unpleasant/ make eyes and throat sting

 

[okeydokey]

Thanks - Ill give it a go this should try and make the whole board the same colour but will give the Liberon bleach (as Ive just been given a half tin) a try in the worst area first.
cheers