could this be the result of too much laquer thinner?
I checked a pair of tefillin yesterday where the shel rosh was clearly damaged by a black liquid like substance that had gotten onto the bottom of the shel rosh parshiyos (see pic). After closer examination I saw that the bottom of the bayis, ie the tip of the mechitzos, had it as well (see pic). Clearly liquid entered the bayis at some stage and did damage. I have seen water damage but this was different because it left a grey / black stain whereas watermarks are usually clear. I think what happened here is that the bottom of the bayis had paint which was washed off with laquer thinner. You can still see some of the paint in the charitsim on the underside of the bayis. The dirty liquid (laquer thinner mixed with the paint) went thought the holes of the tefiros and enetered the bayis, staining the lower bayis and the bottom of the parshiyos. Clearly the guy used too much laquer thinner. I cannot think of any other way these parshiyos would have got damaged in such a manner. The point