I'm going to repaint a ceiling that has a faux finish and a clear topcoat on it. The faux finish is some kind of yellow smear and the topcoat a rather low sheen, I would say satin. Is it necessary to prime with something like INSL-X Stix because of the clear coat, or can I get away with using ceiling paint directly on top?
As for the paint compatibility, the standard advice is to either paint an innocuous area and try it, or do a crosshatch test.
The low sheen is caused by the paint being intentionally "bumpy". That tends to produce good binding to the next coat, so I would expect reasonable results.
The other reason we use primer is as a "barrier coat" to assure chemical compatibility (that the paints don't react to each other or fail to stick). Again a paint or crosshatch test will reveal that.