With all due respect, Duanob, either they weren't pinholes (exactly how small if a pinhole anyway) or you didn't do it "by the book". I appreciate that you had an unsuccessful experience and may be dissuaded from using their product, by you seem to be an outlier in terms of success and satisfaction with their product. Doesn't mean you're wrong, just an unusal negative result.
You are correct about the fuel level causing/promoting rust. But, it's probably easiest to line it (in my opinion) once he's had repeated problems.
Most difficult part of "following the directions" is there's often no way to be 100% certain you've removed all rust, moisture, gotten perfect adhesion, full coverage, etc. it's all about diligence and effort. But still may have to repeat it due to small errors or insuffcient coverage.