I have never had to use this stuff but have read it is one of the ways to clean out rust deposits from the blocked and silted waterways.
I wouldn’t really recommend it as it is an acid and care must be taken using it and disposing of it afterwards.
Cemelex is one of many high strength Hydrochloric Acid based brick cleaners containing specialist additives to boost the action and penetration of the acid solution. Commonly referred to by builders and building contractors as brick acid. Cemelex is formulated with the addition of inhibitors to minimise attack on bare metal and modified to prevent excessive fuming which is widely associated with many acidic solutions.
Best to do it before doing too much dismantling of the engine as you need to keep it all in one place. It is important to read the instructions as to how you mix it, how long you should soak it and especially how you should dispose of it. Afterwards a good flushing out through the system will be required.
If anyone has used this stuff or something similar please let me know how you got on.