HW/Weihrauch HW100 bubbling paint on tank

I am considering bvuying an HW100s. I haven't handled the gun and have some photos. One troubling thing is this bubbling at the fill end of the tank. Has anyone seen this before, or know what this is a symptom of?

I mean, I can figure it's corrosion, and if I didn't get it for a bargain so I can buy a new tank I will walk away, but curious is this is also a symptom of other faults, has anyone else had this happen, or possibly just lack of care from the owner.

I have always wanted one of these, and we don't get many that come available, so I am happy to fix it if I get it at the right price.

Thanks in advance,
Paul.

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First, they are great rifles, and if you get one I think you will like it. That looks like the OE cylinder to me, and the defect appears to be in the finish. I'm not sure of the exact alloy used in that metal. If it is steel it can be polished, inspected and reblued. But HPA has to be respected, and I would probably make an offer with an allowance for replacing the cylinder, unless the owner has an explanation. If it were me, and if the rifle is otherwise in excellent condition, I think a price that would allow replacing the cylinder with an all-in cost of 80% of new would be fair.