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I'm looking to darken primarily the top on a new FP guitar (Brazilian and European Spruce, holly bindings).
I used as dark a shellac as I dared (getting an even color was my priority) but my client wants it darker yet ("old" looking).

How effective is UV at this darkening?

Is the result dramatic or slight?

Thanks for any help.

Best!

You can turn a fresh Spruce top into one that looks distinctly vintage but that might take a full week or so, 24/7.
Unfortunately putting a finish on it first is REALLY going to slow it down. It will do it but you might be looking at several months (probably longer) rather than 2 weeks.
I can provide examples of how dark you can get it without finish but I've never subjected them to UV after the finish is applied, at least not for ageing purposes anyway.

I'd love to see a pic or two Michael if you can post or send them to Laplantedp "AT" aol "dot com"

Here you are David. One bare Spruce, the other had 2 weeks 24/7 in the can. It does have some finish on (applied after) but it's a very clear finish, no tint in the varnish. Done on one single 3 ft. tube.

That is impressive Michael,
I saw your technical info on DelCamp for setting one of these booths up.
Sounds as if because it is an enclosed space the biggest problem is heat build up.........would a larger space help prevent this?

They vary. Some people use different types of lights although the basic idea is very simple. There are a few things to take into account though.
I bought a small used clothes cupboard and adapted it for use as a UV cabinet. The internal space is 50" x 19" x 19", all internal surfaces lined with baking foil. There's no need to use the exact same dimensions. I have 2 x 3ft BL350 (Blacklight) in each far corner - not BLB lights. These are the visible UV lights with the intense Blue light, the same type that are used to attract flies in food establishments.
I only ever switch one light on. Switching both on resulted in the temperature rising too high in the box. I have to place a baby bottle warmer, steaming away in the base of the cupboard. Even using the one light has a big effect on the humidity. Both temperature and Humidity should be determined before placing any instrument in the box. I have a few air vents, 3 x 2" holes near the base and 3 x 2" holes at the top - all baffled so that there is very little leaking light.
You also need the correct transformer/ballast to match the tube that you are using + the starter. I think the ballast also acts as a current limiter. They get fairly warm so I mounted mine on the outside of the cabinet.
If you can place these in an outbuilding it is a little safer, although I don't have that option. They really are nothing more than a fluorescent strip light. I do have a couple of smoke detectors placed above the cupboard, just for some peace of mind.

Almost certainly. Just a few more inches on each dimension will help. It will still need to be vented. I did place a computer extraction fan in my cabinet but although it dropped the temperature it also had the effect of sucking all the humidity out.
That's when I went to using just the one light and I did away with the fan. I have it so that the tanning is a bit slower but the temperature and humidity is OK or at least not bad enough to induce cracks and flat plates. I did have a back that badly distorted when I was running two lights. If you place a Hygrometer/Thermometer somewhere near the top of the cabinet you should get a fair idea of what is happening. Give it a couple of hours to reach some sort of equilibrium.

Again,
Many thanks Michael!

UPDATE-
So, last weekemd I delivered a test piece to "Cue" Gerhard at Gerhard Guitarworks in Windham NY (about an hour from me) and he'll be exposing it in his UV booth. His lamps are the strong UVA-B type used in commercial tanning booths (15 minutes, stick a fork in yourself because you're done....) so are presumably much more intense than the UVA Bug lights. In a few weeks we'll see if we have any tangible results.
Here's a shot of his booth: