Lou asked me if I had anything to say on this topic, and I do.
The short answer is that there should be no reaction, that I know of, with any body oils or acids and any lacquer after the solvents have totally evaporated. There are several reasons for problems with a lacquer finish, including it turning "black"; and all of them have to do with the solvents not being totally evaporated.
We use lacquer because most of the time it is the easiest finish to use, but when there are problems with it, we realize that it is the most difficult of finishes to do well. The reason for most of these problem is the high solubility of almost everything in the solvents
There is always the possibility that the thinners in the lacquer are reacting with something in the wood, something we have put on the wood, or the wood itself. We can't do anything about the wood itself, but we can control what we put ON the wood.
Sanding dust is always something that can be absorbed back into the surface film before the solvents have evaporated. This becomes more of a problem as we sand to a finer grit because the particles are smaller.
Some will advocate cleaning the wood with lacquer thinner or Acetone to remove sanding dust and any excess natural oils before application of the finish. However, this solvent must also be allowed time to evaporate before application of the lacquer. Not allowing this solvent to evaporate can be worse that not using it at all, because it will bring thecontaminates to the surface where they are held in suspension and easilly absorbed into the finish that is applied. And, that can happen to other finishes that are applied after cleaning the wood with lacquer thinner.
The best approach is that there should be nothing on the wood under a lacquer finish other than a CA glue used as a grain filler; and we can get away with using CA glue only because it cures quickly. Everything else can cause a problem if it isn't fully cured or the thinners and solvents have not evaporated. The rule here is that, if you can still smell it, it hasn't evaporated yet.
The rule for lacquer is, wait at least a day before assembling the pen and then do it carefully and protect the pen with a soft flannel cloth or soft cotton gloves, wait at least a week before buffing the pen, and wait at least a month before using the pen. Double these times if the surface film is thick, such as from a dipping application or multiple spray coats.
Unevaporated thinners can absorb acids and oils from our fingers into the lacquer. This can darken the surface film, and the oils will soften the surface film and make it more easilly damaged during use and handling.
Fine surface scratches can be imprinted into the surface of the lacquer if it is handled too soon after it has been applied. These scratches will then become filled with body oils and grime where they can react with any unevaporated thinners.
Buffing the lacquer finish too soon will create even more surface scratches than handling.
Lacquer should be a hard and durable finish. If you have a lacquer finish that is soft and easilly damaged, you are not waiting long enough before handling or using the pen. If you are having a problem with a darkening of the lacquer, it is because you didn't wait long enough before handling, or there is something under the lacquer that is causing the problem. I know of no other reason for a lacquer turning black with age and use.
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