If you already have epoxy primer on it, I would sand that and apply the filler on that. I believe the epoxy will give a stronger bond than the 2K urethane. If you are planning on applying that urethane on bare metal, then you want to check and make sure it is designed for "direct to metal" applications. Why don't you just go someplace else and get epoxy? Urethane and epoxy are not generally interchangeable.