I agree with much of what your saying, especially the wax and grease remover to be blown out of the cracks. Depending on the crack, I don't know if I would use it at all for exactly the reason you stated. Wipe and the bumper down with a dry rag, and then do your repair, eliminate the E&GR completely and you don't have the problem leaving it in the cracks by accident and having a future adhesion issue, that's why I never suggested it.
Bridging epoxy or any over cracks is an option, however, the hair line cracks reappeared where excencuated after the first coat of epoxy and didn't fill them, and without seeing the bumper, would tell me that they are a hairline crack as the epoxy he's using is thick and in most cases should bridge a crack line. I don't like bridging at any time but, the bumper wasn't stripped and here we are.
Your right, putty will has a tendency to sit on top and break if it gets hit and that's the scary part of not stripping a bumper with original paint issues. You or I don't know how thick the hair line cracks are, if they are really tight, and he is using a an epoxy primer that has a fair amount of build, chances are it's not going to flow inside the cracks leaving air trapped in the crack and when warmer weather hits the air trapped in the crack will expand and may become visible, or over time could cause a lack of adhesion (depending on the length and depth of the crack which we don't know without seeing it) you can over reduce the the primer and give it a better chance of flowing into the cracks, but then you come back to the original problem of solvents being trapped (similar situation to W&GR only in this case you can't blow them dry) in the cracks making the crack line reappear later. When paint with defects aren't prepped in a way that entirely removes the defect, these types of situations arise and people must make a judgement call. How wide are the cracks, How long are the cracks, will epoxy flow into the cracks or bridge the crack, putty may not get into the crack or maybe it will, how thick is the epoxy and what happens if I over reduce it to get it to flow into the cracks, will it come back to haunt the Camaro down the road? These are all valid questions that could have easily been solved by removing the questionable substrate prior to applying a primer. But that's where we are now.
I also agree that we don't know the properties of Summit"s products which makes giving advice even more difficult, all we can do is draw from past experience and hope the problem doesn't haunt in the future.
I very much appreciate the fact that you never said "this is what I've done" suggest a repair and continue with '"and I've never had a problem". That kind of advice scares the crap out of me when I know that any solution to situations like this are not without chances of a consequence.
I suggested putty as a bridge and attempting to force the putty into the crack rather than filling it with epoxy because I don't have the part in front of me, I weighed out the possibility of failure and made a judgement call. I felt the chances of flowing primer into a tight crack would bridge the crack without penetrating leaving air trapped was a great possibility and creating a problem, over reducing the epoxy so it flows into the crack has the potential to trap solvents, creating another problem, filling the cracks as pest as possible by forcing the material into the cracks and then applying the epoxy primer, in my thinking, would minimize a problem that is still potentially there. When it comes to bridging a problem with primer or filling a problem like this with putty is not the proper way to eliminate the problem, all we can do now is do our best to minimize the the potential of failure in the finished product...Wouldn't you agree?
I do understand that bumper covers with cracks are a PITA and the best solution is to strip them which also is a PITA. So today we are faced with how to find a solution that hopefully fixes a PITA.
Wouldn't you agree Dead?