I believe plain white vinegar is typically 5-6% acetic acid (appx 0.8 molar). Dr. Pavlis said he diluted white vinegar to 50:50. On tough jobs he might descale repeatedly, with less than 1 hour soaks. For helping to dissolve scale in the pressurestat pipe he would cycle the machine up to pressure a few times - see this post: La Pavoni Europiccola not heating
That makes me think it's not carbonate scale, which should have dissolved in that Oust (citric acid*) descaler. I wonder if maybe you're dealing with calcium sulfate scale, or even worse, silica. One way of dealing with that is to start using very clean, soft water that has no sulfate or silica, and let that white stuff slowly dissolve as you use the machine. Here's some discussion about that: The chemistry of scale in espresso machine boilers [FAQ]
*Edit addition: The liquid 'all-purpose' Oust descaler is, according to the MSDS, lactic acid rather than citric acid.
Thank you for that. I am / was also concerned it might not be regular scale... we have quite hard water, we get scale around the taps/shower, in the regular kettle, but it also seems to come off quite easily with cleaning products.
Most of the scale has came out of the La Pavoni, would it be weird for it to have two types of scale? It is really just right on the base, and on the coil that it's stuck. But even then, it is fragmenting. The scale on the coil has cracks and flecks on it now, it is budging, just so slowly. So it is probably the Oust...
On the bottle it says "contains critic acid", but if you say it's mostly lactic acid based, that might explain things... I wonder if you could just use lemon juice...
After multiple rinses and a mostly clean machine, my coffee this morning was beautiful.
But I still want to kill off all this scale before I return to regular use... hopefully the vinegar will sort it out...