Adrian,
That is perfectly feasible.
You can use some liquid aquadag or for example a spray can of Graphit 33.
I had some Graphit 33, but the propellant was gone over time.
I opened the can and used it as a liquid aquadag which I applied with a brush.
Or, perhaps, for this CRT I used some aquadag that I still had - can't remember.
It is not critical at all.
It is not required to remove all the old aquadag, just remove what is not attached to the glass anymore.
If you remove everything and use a spray can, of course you get the best optical result, but the working is the same as "patching up".
A CRT with almost all aquadag gone:
I masked the neck and around the EHT connector with tape:
And the final result (not a very clear photo due to the black surface):
I let it dry at room temperature, which went fast enough.
Afterwards, just give it a wipe to remove the top which always gives off a bit.
If only a small part of the layer is bad, just remove the loose parts and it is still good enough.
Hope this helps.
Jac