The recommendation is a good start, but more importantly where is the moisture coming in from.
If you fix the cosmetic issue, it will undoubtedly come back if you do not stop the moisture from coming in to start with.
Is it in the area of the chain locker?
Is it close to the screed material in the sheer or bulwark of the hull
Both areas are common locations for rust blisters
on ferro-cement hulls.
migrates in a downward direction (above the waterline ) so you should be able to see if you can identify where the moisture is coming from in a line directly above the rust weep.
A very common failure point in keeping moisture out of the hull
work is underneath the cap rails if you have them.
Amazingly, most designs that I am familiar with use some type of angle iron or flat plate to develop the screed line. The very thin layer of cement ( 1/8" to 1/4") eventually allows moisture to hit the iron below it.
Especially if the builders were not cautious and used insufficient bedding material around all the bolts, or if the cap rail material was not sealed. I have traced rust blisters
up to a couple of feet away due to this type of a failure on a number of hulls.