Lightening a too-dark dye job
Is it possible to lighten a dark dye once it's been applied? I have used a General Finishes water based dye to stain some plywood shelving. It's darker than I expected. Another piece, which I had stained with 80% dye/20% reducer came out much more to my liking. I'd like to lighten the first piece I did, but I don't think I can sand it, since there's just a thin layer of veneer on the plywood.
Yes, there is an "un-do" here, because you used a dye rather than a stain. A dye has no binder, and can thus be modified long after application. Look at the stick in the picture. I applied water-based blue dye and dried it. Then, I wiped the lower end with distilled water—the solvent for water-based dyes. Most of the dye came off on the rag, leaving just a tint on the wood. Note the blotching, due to the fact that the wood was neither sanded nor sealed before dyeing. It was just a scrap from the bin.
Dealing with gray stains
I'm making a pair of boxes from some curly maple. It's beautiful wood, but it has some gray mineral stains blotching it (they go all the way through the wood). I was thinking of using a honey-colored aniline dye over the entire surface to even out the color. So a) what do you think of that plan, and b) should I apply a thin coat of shellac before staining, since the alternating grain will soak up the dye differentially?
You say "mineral stains". Could they possibly also be biological (mold, mildew, or fungus)? I think I'd first try to bleach them out on an offcut. If they're biological, dilute laundry bleach might help, with only a small effect on the wood. If that has no effect, the next step is a two-part wood bleach.
If bleaching has no effect, you're going to have to try to even out the appearance, and that might mean staining all the wood (except the gray stained area) with a gray stain. Once the stain blends in, you can adjust the color of the whole board. But a dye stain will absorb unevenly if you paint it on, emphasizing the grain. You probably don't want to do that with gray. And a pigment stain will seal the wood, preventing you from emphasizing the grain with your honey-colored dye.
Shellac will even out the absorption of the surface, suppressing the curly grain and making it more subtle. But curly grain is what you're paying for in that board. It would be better (IMHO) to stain it with a very dilute honey-colored dye stain, building it up until you get the effect you want. Don't try to hit it in one application.
Once you get the overall effect you want, seal the wood with shellac. Alcohol in the shellac might lift the dye stain, so use spray shellac that won't disturb the surface below. Then evaluate how much of a problem the gray stains present. If they're still objectionable, apply a dilute gray pigment stain to the areas that aren't gray, to even everything out. Then build up enough clear coats to provide protection.