hey. just use these Gaggia bottles 3 for $20

https://www.amazon.com/dp/B01LIEI7TM/ref=twister_B07DFHGBRF?_encoding=UTF8&psc=1

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make sure you choose the 3pack option

Not build up but etching of the aluminum.

Not sure if it's significant etching. Many people use citric acid without complaint.

I use Gaggia's descaler which is lactic acid and what the factory recommends.