This is a question that has been bantered about since the creation of the automotive aftermarket refinish systems started. The most important part of any restoration is the preparation of the metal to form a solid foundation that withstand the rigors of all the elements that the top coats are exposed to. It goes without saying that what is underneath your paint is what will allow your finish to stand up over time and what you put down as a foundation will determine the longevity of your finish.
I have used and tried all types of metal preparation materials from vinyl wash to etch to epoxy primers, these products have evolved and are better today than they have ever been. The debate continues, some people claim that an etch primer topped with a 2K primer is the best way to go, others claim that an Epoxy primer topped with a 2K primer will out last any and everything on the market. Many technicians on this site claim that SPI Epoxy over bare metal is hands down the best way to build your substrate to allow for that great finish.
Up until recently I had been an advocate of PPG's DP line of Epoxy primer, top coated with a high quality 2K primer gave me the substrate I required in order to be confident that I was using the absolute best method for my customers. Over the past month I have had an opportunity to use some SPI Epoxy primer over bare metal, before I tried this product, I discussed what I had read on this forum with others in the industry and in one of these discussions an old customer of mine offered to supply me with some SPI Epoxy and let me try it for myself, (He had been been successfully using it for almost 2 years).
The way I started testing the product, as I try and do with any new (to me) product is to abuse it to see how far I can push it. I made several test panels (18" X 18"), roughed them all up with 80 grit with my DA. The first panel I then ground down with a 24 grit disc and applied 2 medium wet to wet coats of Epoxy allowing proper flash times and set aside to cure. The second and third panel I took a ball peen hammer to them, on one panel I applied the Epoxy, did my body work to repair the panel and gave it 2 more coats of Epoxy...on the other panel I did my body work and applied the Epoxy. The filler on both panels was about 3/16 of an inch thick maximum. On the fourth panel I soaked the panel in a salt water solution and allowed it to dry over night, in the morning, wiped the rust off with gun wash and applied the Epoxy...(the reason I did this is because often after a vehicle is stripped it sits around for a few days before it's coated and oxidization can occur without you knowing it), then I baked the panel for 1 1/2 hours at 160 degrees.
I found that the SPI Epoxy had great sanding qualities...it sanded virtually as well as any top quality 2K primer.
The two panels that I prepped, damaged and then repaired, blocked out very well, I then tried to get the filler to let go from the steel by hitting the backside of the panel with a ball peen hammer...they both stood up exceptionally well and the panel that was primed and then body worked held the filler every bit as well as the panel with filler to bare metal.
The panel that was allowed to see rust, minimally cleaned, primed and baked does not appear to have any adhesion issues. I will leave this panel for several more months before I sand it down to see if there is any rust occurring underneath the primer.
The panel that I ground down with 24 grit and primed with 2 medium wet to wet coats of primer blocked out...all grinder marks where filled and I have based and cleared the panel. It has been painted for about 10 days, it has not sank, the clear has not died off and it looks as good as the day I painted it. (I would not do this to a customer's vehicle, I did this to see how far I could push the product and would not recommend this procedure on any vehicle).
The last two panels I will let sit for several months but, I must say, from what I have seen so far I am more than impressed. I feel I can now go from bare metal to primer to block sand to paint. With all of the other Techs that have been glowing about this product and have used it for years, I feel that I too would highly recommend this product.
Oh, and Barry K, if your reading this post, I look forward to trying your base and clear as well...Both of the early Ford's I'm doing will be SPI.
I hope this helps.