This is one of the reasons why I don't like leaving old paint on a vehicle that has a paint issue. I know it's a cover and they are a PITA to strip, but if they where stripped this would be a mute point. Without seeing how deep or wide the cracks are and the fact that it's got 1 coat of epoxy on it already and epoxy is basically a primer that is glue with rust protection in it, I would say that if you open up the cracks your asking for more trouble, the paint underneath cracked for a reason, it got hit, clear got brittle and no flex agent in it, (you mentioned that the bumpers had been repainted at one time and often shops don't add flex agent into the clear for bumpers and when they get tapped, the paint cracks). After you've roughed up your bumper with 180, take your catalyzed putty and your spreader and apply the putty length wise over the crack. It will in all probability make a raised line where the crack was. Then I would move the spreader and with putting pressure on the spreader go side to side until the line seems to disappear...let it set up and sand it down flush to the epoxy primer. You may have to repeat this on a few of the cracks, but when you have them filled, apply more epoxy primer, block and paint. This should get rid of your cracked cover...is it the right way, well it's a fix for what you have, the right way is to remove the damaged paint.
Let me know how it works.