I use slip plate to replace failed aquadag.
https://www.amazon.com/Slip-Plate-Pl.../dp/B005ESITFQ
One can will do several 20" CRTs. The process to recoat is fairly straightforward: remove neck hardware from CRT and remove CRT from set, place CRT face down on soft material to prevent screen scratches, scrape loose dag off with stiff brush, clean bell with glass cleaner and repeat brush down, mask off screen and ~2" perimeter around HV connection with blue masking tape and paper, also mask off 2" perimeter around neck on bell of CRT (can throw plastic bag over neck and tape that down), then coat the bell with slip plate the way you would if you were spray painting it, let dry and reinstall into TV.
The original dag had perimeter boundaries...if enough of it is intact for you to accurately discern those boundaries use the original boundaries as the masking edges instead of my crude mask here and 2" this and that advice above...the dag should all be connected to it's self there should not be any islands of dag.
Do not replace your CRT over dag flaking... suitable dag replacement material is being made, but new CRTs compatible with sets of that era are not made and will not be made again.