Running into the exact same issue here in Europe on my S14 Silvia Q's:
Took the suspension apart for renovation/upgrades, and since my tank wasn't leaking yet I wanted to do the preventative measure of spacing out one of the straps and adding some foam padding. Unfortunately the above picture is what I've found the tank to look like.
After doing some research I was set on trying the method described in Kingtal0n's thread using the products from tb-bonding.com, unfortunately they do not ship out of the US/Canada and as apparently it's illegal to ship adhesives internationally this was not an option for me.
https://tbbonding.com/product/ptfe-patch/
I've found a company locally that sells a 2K adhesive that they say sticks to HDPE and PTFE, so what i'll try and do is dremel out the cracks, fill the groove with adhesive, stick a sheet of thin PTFE on it and then cover that with some rubber. Effectively recreating the PTFE tech-patch that tb-bonding sells. I'll make a thread to show the process once I've found out if it works as it might help out others with the same issue.