I have some American Cherry boards. I planed and sanded them enough that I removed most (but not all) of the sun-darkened surface.
I'd like it to go back to a nice even dark color eventually. Should I leave it in the sun for a few days / weeks before finishing it or will it still react with UV under a finish (assuming no UV blocking finish) and darken again naturally?
Also, does the answer apply to all types of wood?
Should I leave it in the sun for a few days / weeks before finishing it or will it still react with UV under a finish (assuming no UV blocking finish) and darken again naturally?
Cherry will naturally darken with exposure to UV light, regardless of whether you've finished it or not. My parents have a cherry bedroom set that exhibits this characteristic, even though it's kept in a bedroom with the windows closed most of the time.
However, if you want to "artificially" age it, leaving the piece in the sun for a while (a few hours, even) will help darken it. Depending on your local climate, you may need to leave it in the sun longer. Just be aware that if you leave it on your driveway for a few days, you'll likely have to clean off the dirt and grime that accumulates, which could lead to odd flecking in the wood from the spots that were covered up.
Also, does the answer apply to all types of wood?
Not necessarily. Cherry is one of the faster-reacting woods that is available in terms of darkening with sunlight. Others take much longer to darken/change color.
Personally, cherry is the only wood I would consider artificially aging with this process, and any others I'd just let time take its course and age it naturally.
Should I leave it in the sun for a few days / weeks before finishing it
Given the right conditions only a couple of hours is apparently enough to replicate the natural indoor ageing of cherry over some years. But others living at a more northern location might need to leave the wood (or the finished project) exposed for longer. Here are some test pieces exposed to "direct sunlight for two weeks", bare and with various finishes on them:
[Source: this thread on Woodweb (see note bottom about chemical ageing)]
or will it still react with UV under a finish (assuming no UV blocking finish) and darken again naturally?
As you can see from the above the effect appears to be accentuated under finish rather than slowed in any way (as you say, assuming no anti-UV agents in the finish).
Also, does the answer apply to all types of wood?
Very much not. Usually the effect is a little slower but the same direct sun that attractively darkens freshly-worked cherry often will have a negative effect on other woods.
Just very broadly, lighter woods tend to darken with UV exposure and darker woods will tend to be bleached.
The bleaching effect is especially notable with woods that are any sort of reddish colour, the natural compounds responsible for this colouring in plants are not lightfast and you can get significant lightening and an almost complete loss of the vivid colouring that makes the wood special.
A particular example worth noting is purpleheart, UV exposure will cause the colour of purpleheart to change to its default meh brown colour. But actually this is common with all violet-hued woods, none of which retain their colour indefinitely. So, if you want any violet colouring on a project you should rely on dyes and not on the native colouring of the wood. In addition to having much more confidence that the colour will remain as you intended it to be another advantage this offers is that you can use a pale-coloured wood as the base that will often be much cheaper.
If you would like to experiment with chemical ageing of the cherry, there's no reason to use anything remotely as dangerous as potassium dichromate. Common household lye of course is also a hazardous chemical if used carelessly, but good news is that similar colour changes can be possible using other mild bases. The two most common that people experiment with are simple household chemicals: baking soda and washing soda.
Even in pure form both are safe for direct skin contact so these are much more user-friendly to play around with in the shop.
I've "sun tanned" cherry before by leaving it out for a few days (~6 hours of sunlight for two days). The effect was noticeable, but not overwhelming (e.g. it darkened nicely, but certainly didn't look like a 50 year old cherry table). Clearly leaving it out longer will have a more pronounced effect. In general though you'd just be accelerating what would otherwise happen. Obviously some finishes attenuate UV/light penetration more than others, but you should expect to see some degree of darkening no matter what (assuming that your piece has even moderate light exposure) over time. The question is how dark and how long. Sun tanning will certainly help accelerate that process.
