At this point you should have any dents on the panel filled. If you have any scratches or chips that need to be repaired, you can do that by feathering them out using a block with 180-320. You can use a DA sander but they will not get the feathering tapered as well as a block.
The first thing you need to do is called back sanding. Back sanding allows you to prime your body filler and feathering, but in the case you go a bit further with your gun (which you will need to do) it ensures your primer lands on a sanded surface. Use 320-400 grit paper and sand a few inches beyond your feathering in all directions. Blow off your panel, clean it and mask it up for primer. When masking for primer allow yourself more room on the panel then you plane to prime, do not prime all the way to a solid edge!
Choosing a primer
1 Part Etch Primer
2 Part Etch Primer
This is a low end etch primer, more suited for small breakthroughs rather than large metal areas. Avoid this whenever possible.
This is a high quality metal primer that offers great protection.
Epoxy primer has several use’s. It is primarily used as a metal primer that offers great protection also. It can also be used as a primer surface to provide build and can be sanded and top coated with paint. It can also be used as a non sanding sealer later in the paint process.
This is an extremely high-build primer, used for severe filling needs. It can be applied direct to metal but usually only over small areas. It is recommended to use either etch or epoxy under this primer. Some can be top coated with paint directly, some require another primer beforehand.
This is the most common surface, sometimes referred to as 2K.
It provides a good build over body filler, sands well, dry’s quick and can be directly top coating with paint. It can come in direct to metal varieties but all urethane primers are created different. Some can also be used as a non-sanding sealer later in the paint process.
There is a bit of debate on the best choice but let’s say you have a little bit of bare metal and somebody filler to primer, here are a few possible combinations:
Non-Sanding Urethane sealer
A non-sanding primer is used immediately before painting without the need to sand beforehand. It can increase the durability of the job and/or help with paint coverage.
When choosing a primer, things to keep in mind is the resin technology. Polyester tends to be one of the cheaper resins, more shrinking and not as durable. Urethane is in the middle and Epoxy is at the top. Epoxy has a unique cross-linking technology that makes it quite durable. Etch primer is quite effective on bare metal and either etch or epoxy would make a great choice of metal treatment primer.