PPG states "Chemical treatment or the use of a conversion coating will enhance the adhesion and performance properties of the finished system" That's a pretty clear advice. They also give alternate spec for sanding. All processes include washing and degreasing with wax and grease remover.
Their metal cleaner/conditioner is a phosphoric acid etch system that gives the metal tooth. You can prepare by sanding per specs. You can enhance adhesion by using their cleaning/etching of bare metal for superior results. Dupont and Sherwin Williams offer similar metal preparation products that work the same way.
You can spray an etch product and then spray epoxy. You cannot spray the etch over body filler though because it won't stick. Etch would help epoxy adhesion same as the metalprep steps. An epoxy sealer or other coating needs to be sprayed over the etch primer within 24 hours. That is the open coat window. Etch is not moisture proof, so it needs a barrier top coat applied within a reasonable time. If you miss the open coat time, you have to sand the etch before continuing. If your project sits a long time in moisture or gets wet while it's only primered with etch, it may be failing underneath because it's not water tight like epoxy. Etch is good product but it has a limited range of application for a weekend car restorer.
The product sheets are online. When you follow them, you will get good results.
PPG epoxy stays "open coat" for seven days so it will form a chemical bond your next coat. While it's open it won't sand and you'll gum up your paper. It won't sand because the chemical reaction is still incomplete. It is designed to be gummy so the chemical is open to bond with another coat for the next seven days. That doesn't mean it's junk, it means you missused it.
If you miss the recoat window, you need to let it fully cure and then sand per the product instructions. Then you spray another epoxy coat followed by your topcoat. That topcoat must be sprayed within the new seven day open coat window. If you ignore this specification, you loose the chemical bond between coats. That can cause latent paint failure. It may look great and fail later on.
For sandable primers, spray on K36 or K38 high build products over epoxy which is in the recoat window.
PPG, Dupont and Sherwin Williams have comparable product lines. All three make decent products.
For my build
1) strip to bare metal
2) weld all holes
3) sand, strip, clean and then etch with DX520 and DX579 metalprep then wash/rinse and clean with acryliclean. Tack rag dust off.
4) primer with DPLF epoxy, your coice of color
5) sand off areas to do body work and complete all repairs
6) scuff the whole car per scratch desired per the product sheet.
7) epoxy with DPLF epoxy, 2 coats.
8) while epoxy is open, spray high build as needed, (K36 or K38) Add as many coats of high build as are needed to block the car to ones satisfaction.
9) block sand. Repeat step 8 where spots need attention.
10)spray sealer, such as epoxy mixed for sealing.
11)base/clear or single stage Follow the open coat window for base and clear.