I'm glad to hear that I'm not alone The duration of my experience of DIY activates is similar to yours, as, it sounds, also is our experience of the behaviour of plasters/fillers etc.
The only recollections I have of any fillers behaving as as been described relation to interior grade(and maybe, to some extent, 'exterior') cellulose-based products such as Polyfilla.
Indeed so. As I've said and implied, I was uncomfortable finding myself effectively 'arguing' with a professional about matters which I presume are within the sphere his expertise and experience. However, some of the things that have been said just don'y seem to correspond with what I have personally experienced over many decades, and the major performance difference between two specific products which has been the basis of much of the discussion seems to relate to two products which actually seem to be pretty similar. I therefore keep on coming back to the thought of "what am I missing?" - and have yet to get an answer
Your comments about chemistry made me wonder whether that might be at least of "what I was missing", but I actually think not. As you say gypsum ('plaster') is calcium sulphate dihydrate [ i.e. CaSO4.2H2O ], and that is what the Toupret Interior Filler is said to contain. However, if one heats that under the right conditions, one loses some of the water molecules and ends up wth the "hemihydrate" [ roughly CaSO4.½H2O], wich iis what the Easifill products are said to contain - and which, as you've said, is otherwise known as Plaster of Paris. However, by understanding (not guaranteed to be correct) in that when one mixes the hemihydrate with water, it rapidly turns back into the dihyrdate - so I'm not sure that the fact that the two products contain (as supplied) different hydrates makes much difference to what one has one it has been mixed with water!
However, it seems that it gets more complicated than that, since there are apparently at least two variants (alpha and beta) of the hemi-hydrate - but I am then totally out of my depth and don't have a clues to to whether that has any relevance to what we've been discussing
In another attempt to resolve my confusion, I wonder if it is possibly that (rather ironically) professional decorators sometimes (maybe often?) can't afford to wait for the amount of time ideally required for 'complete curing and drying'of fillers etc., before finishing (including 'rubbing down', if necessary and decoration? - and interval that can vary from 'a few days' up to 'a week or three'!?
I have conceded that I was wrong to recommend the interior Toupret filler in cases where there might be a leak further down the line.
With regards to drying times. If you try to sand a filler that has not cured, you will soon find that your sandpaper has clogged. In the event that it does not need to be sanded and you paint over wet filler, the paint would remain wet until the filler dries.
If I have time constraints, I use 2 pack polyester resin filler. The deeper the hole, the faster it cures. I can fill, for example, a 35mm light switch back box and sand it about 30 minutes later.
In the case that I referred to previously, the Easifill was dry enough for me to sand. It was dry enough for me to apply a dilute coat followed by 3 full fat coats of emulsion. All was good until the leak made the paint blister where the Easifill had been used. The emulsion on pre-existing skimmed plasterboard simply yellowed and did not blister.