The main benefit of epoxy is that it is not porous. So having it on a body and driving the car is not a problem even if it gets wet.
Shooting epoxy or a filler primer does mean you have to have the correct size needle in the gun. You should have at least a 2.0 gun. Anything smaller and you will have problems with flow.
I spray a cover coat on any bare metal area that needs bodywork(bondo). After getting the areas smoothed and blocked ,I sand the whole vehicle with 150 grit, then 220, then 320. You can do this wet as well.
Then I apply 3 good coats of filler primer,letting it flash off for 30 minutes between coats, which will fill in any remaining scratches.
Then I let that dry a good 2 days. Now comes the fun.........block sanding with 400 to smooth everything out. Then after that is done , you need to topcoat the filler primer with a couple of coats of epoxy. The filler primer is a polyester and will absorb moisture. Thats why it needs to be topcoated.
Besides the cost of all the materials you will need, you are also looking at lots of hours in preparation. I had close to 60 hours just in sanding.
It's like I said , the preparation is so time consuming to make it right. That's why a good body shop charges 15-20K for a start to finish paint job.