Many soferim (scribes) are accustomed to write on a parchment (klaf) known as a klaf mashuah (a coated parchment). Towards the end of the tanning process, the parchment is coated with lime, forming a smooth, white layer on the klaf. This process not only leaves the parchment white, but it makes it easier to write the letters, especially on poor quality parchment.
Some Posekim raise halakhic objections to this practice. For example, is this whitewash considered to be a haziza, i.e., an interruption between the parchment and the ink? Also, the letters are more prone to peel and crumble, especially when touched by the pointer, which may invalidate the Sefer Torah.
The Panim Meirot (3:32) writes that this is the preferred practice, a "hidur," i.e., because it beautifies the mitzvah. Furthermore, since the substance is painted onto the parchment to improve the quality of the writing, it is not considered to a haziza. Therefore, he writes that it is preferable to use a klaf mashuah.
R. Ovadia Yosef (Yehaveh Daat 6:55), however, writes that one should preferably wipe off any excess lime substance before writing, although he does not invalidate a Sefer Torah written on a klaf mashuah. He adds that while there is sufficient reason to be lenient regarding a Sefer Torah, especially in light of the view of the Rambam (Teshuvot 294) who permits reading publically from an invalid Sefer Torah, it is proper not use a kalf mashuah for tefillin, which is a Biblical obligation.
Finally, since the letters written on a klaf mashuah are slightly elevated, the person reading the Torah with a pointer (yad) should be careful not to chip or peel off the writing.
