Every time I find a thread asking this question I respond by saying the razor blades and vinegar methods are a waste of time and to use acid. Well I finally took some pics of a tank cleaning to use as evidence.
Here is the product I used:


Here is the tank before:

I sprayed the tank down with water, sprayed on the acid, wiped with a rag, and rinsed.
Here is the tank 5 min later:

Here is the tank dry:

It took longer to dry the tank than it did to clean it.
So there you have it, indisputable evidence of how quickly and thoroughly a mild acid will clean a used tank.


Here is the product I used:


Here is the tank before:

I sprayed the tank down with water, sprayed on the acid, wiped with a rag, and rinsed.
Here is the tank 5 min later:

Here is the tank dry:

It took longer to dry the tank than it did to clean it.
So there you have it, indisputable evidence of how quickly and thoroughly a mild acid will clean a used tank.

