Aaaawwwww man I’m a new member of this club so have only just read this post my rt had exactly the same issue it was my dads bike and he kept it outside under covers !! When I inherited it after he passed I decided to restore it and the only way to fix this is to have the tank stripped to bare plastic and start again. The painter who did mine said it was something to do with the original primer they used possibly not correct for use on plastic or not applying adhesion promoter first off !! Hope your issue is sorted now