More chips

I suppose an argument can be made that you can get better adhesion by topcoating as soon as possible after that initial dry and not dragging it out until later in the time window. I really don't know how important that is and have never played around with it that close. I have had one chip with the MTK that separated at the color/surfacer to epoxy interface and another one that went to bare, sandblasted metal. The one that went direct to metal did not have a surfacer applied at all. I am usually pretty good at keeping to the time windows so it's anybody's guess why the one I mentioned first separated at that point. What I didn't expect was the one that went all the way to the metal. I am quite sure I did about everything by the book on that one. I can say that my only real experience with chipping paint has been in the last few years and both times with the MTK. I cannot recall any similar problem over the years with either hardened Delstar AE or hardened Centari AE - except the stone/sand blasting that occurs on auto lower rear quarters and some rocker panels from extensive travel on dirt roads. I don't classify that in the same sense as the "chipping" we are talking about here though. I am not a student of this but it would seem to me to be relative to what I think is a harder, more brittle film with the MTK than the hardened AE. I will definitely be trying some Concept higher cost acrylic urethane later this spring when warm weather arrives. I am not going to chip the parts intentionally but may get a feel if any noticeable difference with the MTK develops over time.

I also know that the one chip that happened on my OEM bc/cc Toyota separated at the basecoat color/primer interface - but that is a different type of system than we are talking about, so any conclusion from it is questionable.

I see that the MP 170 tech sheet doesn't specify a flash time between coats at all. Of course the earlier version of the sheet recommended just one coat - which would make flash time irrelevent. The latest tech sheet incorporates a 15 min induction period, which is new to 170, and now recommends 1 or 2 coats but doesn't indicate a flash time if two coats are used. In comparison, the DPLF (which has always had a 30 min induction period) recommends 2 coats if a metal conditioning treatment is not used, with a flash time of 10-15 min between coats. As I said before the time to apply a topcoat is a minimum of 90 min if 2 coats are used - clearly longer than the flash between coats. An overnight dry is recommended if body filler is to be applied over 2 coats of DPLF epoxy. The DPLF has a seven day window instead of the 3 day window for the 170.

I did check out 2 DuPont epoxies:

2540S:
no induction period
no mention of flash time between coats
dry time to topcoat if 2 coats are used = 40-60 min
24 hr max window to topcoat w/o sanding

Nason HS941-35:
no induction period
no flash time between coats
dry time to topcoat even if only 1 coat used = 60-90min
no mention of any max window to topcoat w/o sanding

It seems clear to me that both PPG and DuPont want to get most of any solvent to flash off before proceeding to topcoat. I suspect that if solvents are still flashing when a topcoat is applied, the possibility exists that continued flash can result in additional shrinkage of the underlying primer film and sandscratches showing in the topcoat. But that's only speculation on my part. Perhaps Frank can shed some light on the issue, at least from a DuPont perspective.