Richard,
I hope the following helps. Others on this forum certainly know more than I do about this subject.
It likely depends on the type of varnish to be removed. I have successfully used Rubin, Brilliant and Cristal.
Most of the old varnish used in the interior woodwork of my 1954 home was alkyd type that simply turned to dust when abraded with any type of abrasive, thus not loading up the abrasive paper. But the ?shellac? underneath that varnish would gum up the abrasive paper unless I took steps to avoid heat buildup during sanding, i.e. moving the sander around quickly rather than hovering over the same spot until I reached bare wood, using light pressure against the work, and lowering the speed of the sander, and of course, hooking up the sander to a vacuum.
Beware that many polyurethane type varnishes are likely to become gummy when abraded due to softening due to the heat generated by the friction of abrading. For these types of varnish, I prefer Brilliant, and find it especially important to greatly reduce the speed of the sander to reduce heat buildup and thus making the varnish gummy. Sometimes I also use one of those crepe rubber sticks to frequently clean any buildup off the abrasive on my Rotex disk. Note that I did all of my restoration work before Cristal was available, so I don't know if it might have worked better. Its open coat design should help eliminate clogging / buildup of the abraded old varnish.
Note that many manufacturers of kitchen (and maybe bath) cabinets use catalyzed finishes, which can be very hard and very abrasion resistant. For these I have not found an effective abrasive disk for use with my Rotex 125, and instead use my 4 x 24 belt sander with ~80 grit belts. This type of finish does not clog the abrasive, and does not become gummy when abraded. The time to abrade away the catalyzed lacquer / varnish from one side of a 12" x 20" flat panel with my Rotex was more than 1 hour, using 50 grit Rubin!! In contrast, my time to do the same with my Bosch 4 x 24 belt sander with an 80 grit aluminum oxide belt was about 10 minutes. Grounding of the belt sander to the Festool CT 22 vacuum was very important during this operation; otherwise I received a constant stream of static shocks. This same sander had never shocked me when sanding bare wood, e.g. after glue up of panels.
Dave R.
