"One of our experts like HRP will know for sure though." (QUOTE)

Expert?....Ha! I still learning like everyone else. :-)

I still mud first, but it's because I've done it that way for so long, and it works for me with no problem......however...

The older fillers did not stick to primer and paint worth a damn. Today that has changed, and most of the newer fillers are formulated to stick to sanded paint. Just be sure to read the instructions.