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I bought a used radiator and want to get the rust out, I think this is all aluminum. It is not imstalled.

Lemon juice?

Lemon juice should be fine, it will darken the aluminum in time. I use a blend of lemon juice and dish soap in my ultrasonic cleaner for aluminum and zinc castings.

Also both Phosphoric acid and Chromic acid can be used on aluminum. The Chromic can be blended with 10% Sulphuric acid.
One may need to watch the radiators with plastic tanks, I have not payed any attention to what will harm the gasket or plastics. I know Sulphuric acid will harden up Nylon.

Then back flush with water to try and clear as much crap out the top as you can get.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.