Unfortunately my experience with the spray on filler/primer has not been good either. I used Dupli-Color Automotive 2-in-1. I tend to spray outdoors in a pretty humid climate, and that may be the problem. First time I used it was under their automotive paint. Second time was under my go-to rattle can paint (MTN95). In both cases there were some adhesion problems.

Based on my experience if you are using a tight grain wood, MTN95 does not require primer, and fills about as well as filler/primer. It is no more expensive than most rattle can primers, adheres to the wood as least as well as it adheres to primer. So, I have stopped using primer, and just do an additional coat or two of MTN95.

For open grain woods, I can't recall anyone on the forum finding spray on filler sufficient (me included). I have tried a variety of fillers, and have gotten the best results with AquaCoat Clear. although it usually requires many coats to f

I haven't used TimberMate, but that seems to be the go-to filler for most folks here. Many of the cheaper water-based fillers or drywall compound I have used have not worked particularly well. Sawdust and sealers has worked somewhat, but don't get me all the way there. Bondo works pretty well, but only if a solid color is going over it, and takes a lot of sanding. I am experimenting with Famowood Latex Wood Filler which is supposed to be similar to TimberMate but without the strong smell.

In general the fillers that are solvent based fillers shrink less than waterbased, but give off more VOCs and are a bit more toxic. The water based fillers tend to be less toxic, but shrink a bit more so need more coats.