quote Tha Driver:... PPG epoxy primer is the best. It sticks better than anything else, & it's more flexible than anything else....

You forgot to add:

It's more expensive than anything else.

From http://www.autobodystore.com/ms7.shtml

"Urethane primer (2K) Urethane primer is the most common primer used in auto body and restoration by far. It has good solids and fills well. It is easy to sand and can provide you with a perfect body when blocked properly. Care should be taken when applying it as to not use too much. It can shrink when applied too heavy. It is the best all around primer for applying over plastic body filler and for surfacing your work. If used properly it provides the proper film thickness under top coats and is the perfect substrate for bs/ss and SS.

Benefits:
- Easy to apply, and sand.
- Applies smooth.
- Fills well with minimum of shrinkage

Disadvantages:
- Contains Isocyanates.
- Should always use an etch primer under it.

Epoxy primer (2K)

Epoxy is a good corrosion fighter. It is has a very sticky resin and will provide good adhesion to MOST substrates. It typically has poor filling and sanding qualities (that sticky resin makes sanding difficult) ....
Perfect for frames and components, radiator supports, items that are sandblasted and you only need to prime and paint. You use it as a non-sanding “primer/sealer” and then paint right over it.

Benefits:

- Good chip resistance (it isn’t as hard as a urethane)....
- Etch primers can skipped because of its excellent adhesion and corrosion properties. (although for maximum corrosion protection apply a wash etch under the epoxy)
- Provides good base under plastic body fillers (skip the etch if you plan on using plastic filler over epoxy)
- Epoxy has no isocyanates .

Disadvantages:

- Poor sanding qualities
- Poor filling"

Etch primer is not required for bumper covers.

[This message has been edited by 30+mpg (edited 06-21-2009).]

quote Originally posted by 30+mpg: From http://www.autobodystore.com/ms7.shtml "Urethane primer (2K) Urethane primer is the most common primer used in auto body and restoration by far. It has good solids and fills well. It is easy to sand and can provide you with a perfect body when blocked properly. Care should be taken when applying it as to not use too much. It can shrink when applied too heavy. It is the best all around primer for applying over plastic body filler and for surfacing your work. If used properly it provides the proper film thickness under top coats and is the perfect substrate for bs/ss and SS. Benefits: - Easy to apply, and sand. - Applies smooth. - Fills well with minimum of shrinkage Disadvantages: - Contains Isocyanates. - Should always use an etch primer under it. Epoxy primer (2K) Epoxy is a good corrosion fighter. It is has a very sticky resin and will provide good adhesion to MOST substrates. It typically has poor filling and sanding qualities (that sticky resin makes sanding difficult) .... Perfect for frames and components, radiator supports, items that are sandblasted and you only need to prime and paint. You use it as a non-sanding “primer/sealer” and then paint right over it. Benefits: - Good chip resistance (it isn’t as hard as a urethane).... - Etch primers can skipped because of its excellent adhesion and corrosion properties. (although for maximum corrosion protection apply a wash etch under the epoxy) - Provides good base under plastic body fillers (skip the etch if you plan on using plastic filler over epoxy) - Epoxy has no isocyanates . Disadvantages: - Poor sanding qualities - Poor filling" Etch primer is not required for bumper covers.

So it says you use "an etch" primer under the urethane primer. Of course they're talking about metal. They also say that epoxy primer sticks better. They're talking about general paint info too, not specificly PPG.
On body parts, I always (OK usually) sand it (epoxy primer) with 400 wet. Yes, it's a pain (& you have to wait AT LEAST overnight for it to dry enough - better to wait several days), but it gives you the best job. Best adhesion (to both body & paint), best sealing, least shrinking.
If you have MAJOR imperfections, you can use the urethane (2K) type primer OVER the epoxy, &/or use a catalyst-hardened putty. I always prime over all of that with epoxy before painting.
~ Paul
aka "Tha Driver"

DO NOT TOUCH THE TREUM!