filler makes up for not straight metal, you can straighten metal to a very high level with time, patience and skill. It's typically not feasible to not use filler but that will always be a potential point of failure. Proper prep work and quality products minimize that drastically. Polyester type high build primers are a good option once you get the metal straight or your bodywork if you are using filler. Sealers or DTM primers or epoxy primers can be used but it's always best to use a singular product line and go with their desired sequence. Every body man I've known has their preferred method from bare metal to paint. I'm a big proponent of having straight metal from the beginning and keeping filler to an absolute minimum when at all possible. Look into a shrinking or phenolic disk. You can do wonders with a hammer and dolly and a shrinking disk if you have the patience and desire not to use fillers. It all comes down to the desired results and the effort that is acceptable. None of my comments are meant to sway your project, it's your project, I'm just mentioning some of the things we do or have done depending on the desired outcome.
