I didnt encounter this with the guitar in my avatar pic (body wings quarter sawn sapele, cap quilted/pomelle sapele). At least no more than any other wood. The finish was simply sanding sealer and water white hi gloss nitro. The sapele does have a significant iridescent quality though, more than true mahoganies, resulting in more depth/chatoyance, especially once you put clear coats on it, and especially if you get quarter sawn or figured sapele. I suppose this could be perceived as 'darkening'. Its a good thing though. Prior to finishing i did my final smoothing with fine scrapers to maximize the grain clarity/iridescence.
Remember to take into account the varying colour tones and whatnot when comparing even within the exact same species, nevermind between different species (sapele isnt a true mahogany).
To get an idea of what the wood will look like under finish, Jack, simply wipe it down with mineral spirits and this will give you a good idea what the lens effect of the finish will do for the appearance. In my case the mineral spirits made it appear slightly darker than the actual lacquer did.
I have some chunks of mahogany from an old church pew and some sapele scraps in my stash, if I have time today ill do the mineral spirits thing and pic the results. Mind you Im not sure the variety of mahogany, and its pretty dark to begin with compared to some.
Okay so the top piece is quartersawn sapele, the bottom is flatsawn mahogany. I prepared them beforehand the same way, a quick shot on the jointer and a scraping with the grain to tidy them up a bit. Then i wiped each with an equally damp cloth of mineral spirits. I tried to do a shit with naphtha, which doesnt exaggerate the darkening as much, but i cant get the pic fast enough to get a fair comparison.
The pics not great quality, but i think its enough to get an idea. Also I will note that Im not saying yay or nay, just that I havent seen this in my experience.
No problem. Personally speaking, sapele is a great wood In terms of appearance. The striking ribbon pattern found on quartersawn stock is great and usually very consistent. No stain required! Other than visually there are some differences from mahogany that need to be considered, but theyre not bad things, just differences.