Bill, to elaborate on PPG products further......
Disclaimer:
Keep in mind I am not an expert on PPG products, have not used them much at all, but these comments are based on reading various posts over the past ten years or so by guys who do actually use the product. So if any of you PPG guys do see something needing clarification/correction, please chime in.
Where the PPG rep was referring to not using a paint stripper, PPG does have a metal prep for most all different metals that will promote adhesion with their epoxy (or other) primer. The metal conditioner for steel contains phosphoric acid, which helps in converting any rust issues. I would add here that any loose scale needs to be removed no matter what process you are using. Deep pits are hiding places for this scale, and sometimes the metal prep will aid in loosening some of this scale for easier removal. But IMO, every bit of foreign material, whether that be rust scale or rust converter/rust scale mix, needs to be removed from the bottoms of pits before any epoxy and then filler can take place. Just because someone refers to their product as a rust converter does not mean it's converted rust scale into something that will hold paint and it's ok to leave that crap in there. Neglecting to clean them thoroughly will normally come back to bite you in a paint delamination later. As with any rust repair, I've always said they are cheap to fix while they are still in bare metal. So true in this case as well.
Here's the PPG metal prep document, you would use either the DX520 or DX579 on your steel. I'd say your PPG rep could guide you on which would be best to use...
http://www.bapspaint.com/docs/psheets/PPG/Automotive/Deltron/P-226.pdf After this prep, they recommend spraying with DPLF within 24 hours. This will limit any flash rust due to humidity or other atmospheric conditions. DPLF comes in different colors, determined by the numeric suffix to the part number. As stated elsewhere, DPLF is not easily sanded. This is largely why it is normally topcoated within recoat window with high build, as that is something that IS easily sanded. Also, once cured (if not topcoated) Epoxy seals rather well such that it doesn't offer a good mechanical bond to subsequent coats of other paint products.
Chemical bond... paint is topcoated prior to full cure such that components will cross-link, or attach to each other chemically.
Mechanical Bond... Paint has already cured, but has been properly abraded (sanded, wet sanded, etc) to provide adhesion or grip for the subsequent coat.
Definitions aside, what I have read on the DPLF is that once cured, regardless of mechanical prep, it does not provide OPTIMAL adhesion to subsequent coats. So if you didn't spray high build on top of your DPLF and your project was put on hold for XX months, they recommend that you abrade the epoxy, spray a link coat of DPLF over the first, and then spray the high build after the epoxy has flashed. The epoxy adheres well to itself, so this method will give a chemical bond to the high build, which gives better adhesion in this case.
A couple different scenarios.... Had you sprayed K36 high build on the first go round, abrade it when ready to continue with your project and it is ready to spray with more high build, or sealer, etc. But this paint is also more porous than the DPLF, so if your project would be exposed to outside weather conditions (rain) then it may be best to leave it in epoxy only. About 3 good coats would be needed to seal out the moisture effectively.
