We have heard of a stop-off material called “Sel-Nite” that is used in the nitriding process and were wondering if you had heard of it, whether it worked and, if so, how effective it is?
As I recall, Sel-Nite is one of the less common nitriding stop-off paints although it has been around for a long time. It is an alkyd resin containing tin (Sn) and was intended to be equivalent to, if not in many cases superior to, tin plating. It is a water-based product and comes in the form of both a powder and a liquid that are mixed together (typically 1 part powder to 2 parts liquid, by volume). The mixture tends to separate and should therefore be stirred often and especially right before use. It is usually applied in one thin coat to prevent migration of the material. It reportedly works well but is very expensive. One of the applications in which it has been used successfully is to coat crankshafts prior to nitriding.
A more modern nitriding stop-off material (there are others) is Condursal N623p, which I believe is less expensive and good up to 90 hours at typical nitriding temperatures - up to 1100ºF (595ºC). Condursal N623p contains both tin and chromium oxide, which melt and wet the surface of the component it is intended to protect. It acts as a diffusion barrier to nitrogen absorption.