[Arran]

Arran in the microscopic sense Lacquer is porous simply the nitrocellulose lacquers form a layer by the solvents drying out and the lacquer organic chain structures laying like bricks on top of each other without any mortar. That is why you can use lacquer thinner and dissolve the lacquer even after it has been dried (for many years) that is one of the tricks of the trade to fix alligatoring of old lacquer when no major chips are missing, the lacquer re-flows itself. So any solvent to the least aggressive to the most aggresive will have an impact on lacquer. Since the early forms of lacquer were made with the combination of "gun cotton" (cellulose) and nitric-acid lacquer had some covalent bonding with N(nitrogen), C(Carbon) and O(oxygen) and since water (H2O) can also be classified as a solvent since it will exchange and react with lacquer, so will the outside air and any impurities like nicotine, cooking grease over time (that would be in most kitchens in the 30's-40's) and the lacquer when exposed to direct sunlight over the years the lacquer will photochemically react via IR and UV radiation to break the chemical bonds down also. So there are holes between the lacquer components , not holes visible to the naked eye but holes that are open to small molecular materials, like water, etc. Later coatings usually have a crosslinking function that takes place either by the addition of another chemical component or by the evaporation of the solvents that better seals the surface making it more resistant to the outside impurities.

Not disagreeing with that at all, but it does take considerably longer for water to get past a relatively fresh coat of lacquer verses an older crazed one, both are much more resistant then the old shellac finishes. I don't think that nitrocellulose lacquer is quite as volatile as gun cotton, at least after the solvents have evaporated out, the ingredients may be similar but I have taken off old lacquers with a heat gun and it never violently combusted as it's chemical cousin would have. From what I recall gun cotton was discovered after a cotton rag was accidentally dropped into nitric acid, washed out, dried, and then exposed to a flame. It's been a long time since I have taken chemistry, much of what I know on the subject of finishes was from direct experience.
Best Regards
Arran