I haven't seen any 'solids appearing' or breakdown in the phosphate solution when still unused in the original bottle (probably 7 or 8 yrs since purchased).
I don't recall the dilution % specified off hand, but you add a certain amount to a larger amount of hot water (200 F) to use it. Only rarely have I needed to 'reheat' an already used solution for later use, so I suppose when Lloyd talks about 'breakdown' that's what he's referring to. I only mix up what I need for the prepared parts I have and after treating them I generally dispose of the used solution.
I have a gallon of WD 40 that I use to clean/seal the phosphate after phosphating; I do reuse the WD 40 bath I use, replenishing when necessary.
PS. I purchased a 'set' of stainless steel 'pots' from Walmart fairly inexpensively, so I use the 'best' size pot for the amount of parts I'm doing. I also use an inexpensive 1 burner hot plate for heating the water/solution.