Oxalic acid.

[whiskywill]

I know oxalic acid is used to lighten wood but will it work on this?

I have spruce guitar top which I jointed, cut to shape, inlaid a rosette and sanded to near final thickness a couple of years ago. I was then distracted so it was put on a shelf. It was partly covered by a sheet of cardboard for protection but the uncovered bit has darkened, I assume by being exposed to daylight. Before I splash the acid on it, does anybody know if it would remove this type of discolouration?

 

[custard]

Oxalic Acid is unlikely to be the solution you need, the sun darkened patch will have a slightly soft edge that would be very difficult to align to, the timber will have experienced a colour shift that Oxalic Acid won't match, and the degree of lightening you'll achieve isn't controllable so won't necessarily be the amount you want.

I'd follow one (or more) of three routes. Expose the entire surface to sunlight, given enough time it'll even itself out. Use a two pack bleach on the entire surface followed by a stain. Sand or scrape back the entire surface, sun darkening won't go deeper than a tenth of a mill.

 

[custard]

Incidentally I confront this problem all the time. I use a lot of American Cherry which acquires a suntan in double quick time. Just laying out the components during the course of a build will often see "bikini lines" here and there. So when I say Oxalic Acid isn't helpful I'm talking from bitter experience!

 

[whiskywill]

Thanks for the advice. As it is near final thickness I tried sanding with 400 grit but it hasn't touched it. I will hang it in the autumn sunlight to see if it gets a suntan.

 

[ED65]

As it is near final thickness I tried sanding with 400 grit but it hasn't touched it.

Could you scrape? Obviously there is lighter wood under the surface but 400 is too fine for taking that amount of material off.

Suntanning the whole surface is probably your best bet, it'll be interesting to see how much light exposure it'll take to even up the colour.