POR15 makes a kit which supplies cleaner, etching rinse and sealant. Cost about $60- After cleaning with any of the described methods above, (I have mine boiled out at the local radiator shop) this will make the tank like new inside, encapsulating loose material and even seal pin holes.
The trouble with any tank, even when kept full is the very top is always exposed to atmosphere and will collect condensation. The tank I just finished for example, the top surface was full of tiny pin holes.
