ask your chemical supplier for a "built caustic". It has extra surfactants/ sequestering agents/ additive that make it more effective.

Chlorinated caustic is also more effective, but you must make sure to completely rinse it off because chlorine will pit stainless steel and eventually make pinholes if left overnight or extended periods.

You may be able to add hydrogen peroxide or chlorine afterwards but if you get it from the supplier already prepared it will do a better job.


You may also alternate acid descaling and caustic cleaning as the mineral deposits may make the rest harder to clean thoroughly.

An external boiler can go very long between cleans. Up to 50 brews before each cleaning session is quite possible.
But care must be taken that the steam pressure is low. Under 1 bar preferably. Otherwise, you will get fouling faster.

A standard formulated caustic should work fine. If it contains EDTA, it will do the job.

Pure caustic is a poor cleaning agent.