Sounds like a problem under the lead, but could be the primer used did not adhere to the lead. Sorry to be the bearer of bad news, but the only way to fix it and solve the problem is to start sanding. But like someone else said, try to go one layer at a time. That way if it is just a primer problem you arent doing a lot of work that does not need to be done. I personally remove all the lead when I restore a car, just because I have seen it bubble too many times and ruin an expensive and otherwise perfect car. As for only using epoxy over lead, I will agree it would be the best option on the market, however the cars came with lead from the factory and an enamel or laquer based primer, so I would not say epoxy is the only thing you can use over it. I do recommend epoxy over any bare metal though. Best of luck to you.