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"when the tanks are new they dont flash rust so why does it happen?"When steel is run through the roller mill to progressively bring it to it's sheet thickness it effectively ends "forged" with very high compression and, to a certain degree, refined inpenetrability as the final surface. This is generally pretty resilient to surface corrosion as it's just so compacted, with any subsequent disturbance causing reduction of that finish whether starting with corrosion which once removed have partly destroyed that finish. All subsequent treatments by removing rust can't reinstate that first finish status, at which point it will need some element of protection. What that is I'm not sure in terms of effectiveness, but it can't be just left alone.
Rince out with de ionised water and then swish diesel around inside the tank to create an oily layer.
I use EvapoRust and rinse with diesel. The last thing I am going to put in a freshly derusted tank is water. When you dump out the diesel into a jug let the sludge settle and pour the diesel back into the tank for another round of rinsing. I rinse at least 4 or 5 times. Never have had any flash rust with this method. The diesel can be used over and over again. Just keep pouring off the clean diesel and dump the sludge.