Hi,
regarding firescale (surface) fire stain (sub-surface) prevention, i highly recommend Prips flux as a barrier flux, in conjunction with your flow flux.
search the forum for the recipe
boric acid
borax
TSP
boiling water
each of the 3 ingredients plays an important part in the fluxes effectiveness…ie: the borax adds a longer working time, etc
if i find the post explaining it, i will post
it is amazing…sterling silver remains pristinely clean…
you spray it onto a heated piece, and it frosts over white
i suggest flushing out the sprayer at the end of the day to a oid clogging it up
the best spray bottle i have found so far is an empty Bactine antiseptic spray bottle…
https://orchid.ganoksin.com/t/any-information-on-firestain-really-any-at-all/57336/9?u=wldlzrd1
julie