Bubbling paint is a sign of oxidation (rust on a steel object). To stop it you would have to sand down to the oxidation layer. After cleaning out the area you will need to primer the area, a rattle can can do this. Won't look fresh and new but will stop the oxidation. Left un checked it will continue and get worse. eventually it will crack through the paint. could take a month. Could take a year. It is all a factor of variable you can't know by looking at it.
Sanding will help but is only the first step in the repair. It must be fully cleaned out.
Closed. This problem was from a bad thermostat. Replaced the thermostat and it's good to go. I've got new problems but what else is new. The manual above is highly useful.
I replaced the rear in this manner and the front unit and furnace continued to work. The furnace and each unit are relatively closed loop to thermostat.