mark79,80

I stripped the bumper starting with Bulldog rubber bumper stripper but
could not remove the 2K primer and epoxy so I resorted to using
low odor aircraft stripper. The bumper did stretch when I used the
stripper but seemed to shrink back once I rinsed the stripper off.
Repainted it using PPG NCP 250 primer followed by two coats of
PPG epoxy as a sealer, color coat and PPG 2002 clear. Painted it
on the car with all the bolts loose so there was a gap between the
bumper and the body.
Painted it in February and it looked great until I started to tighten the
mounting bolts. In several areas adjacent to the bolts it looked similar
to a paint defect called line checking. The lines or cracks run parallel
to the edge of the bumper and extend outward an inch or so from the
bumper edge. I the paint felt rough over these line checks.

I removed the bumper and the line checks are almost not visible
and the paint feels smooth over them.

I am thinking that I weakened the structure of the bumper by using
the aircraft stripper so the bumper stretched a bit when i started
tightening the mounting bolts?

crazywelder

I assume since you used Bulldog, you have a urethane fascia and not fiberglass? If so, sounds to me like too much paint build and when you tightened the mounting bolts, the facsia flexed and the paint cracked. If you use conventional body paints on urethane like the NCP and the DPLF followed by more layers of color and clear, you face the chance of paint cracking if the fascia flexes especially if you wait until the paint is cured before tightening. Paint build on urethane is best kept to a minimum to avoid cracking problems and you need to be careful which products you use. Urethane is not like the car body and the paint process is different. When the NCP and DPLF cure, they are not flexible and there is a good chance they are going to crack.

The best process I've found for bare urethane or urethane with old paint that is in good condition is Transtar Hydroflex primer followed by the color and clear. The Hydroflex primer remains flexible and is designed for urethane fascias (adhesion and flexibility) and can be built up and block sanded which is what I think you were striving for with the NCP. Adhesion promoter would be another choice over bare urethane but you can't block sand it. With Hydroflex, your color coat is applied over it with no sealer and then followed with clear so paint build is kept at a minimum.

Obviously fiberglass fascias would not have the same paint cracking issues if they are mounted correctly. However, if you just slap them against the body and tighten them down, fiberglass fascias can have the same paint problem if the two mating surfaces (body & fascia) don't match each other. As you draw the fiberglass fascia into the body, the fascia flexes to match the body and the fiberglass distorts and the paint can crack. This is why it is so important to fill the back mounting surface of the fascia to match the body shape to eliminate the flexing.

Urethane fascias are designed to flex, so before you start the paint process with urethane fascias you need to decide the order in which you will paint and mount the part. The order you prefer will determine whether to use flex agents or not. The tech sheets tell you which paint products are compatible with flex agents (DBU and DCU are). If you paint the fascia off the car you should use a flex agent. If you paint it on the car you don't necessarily need flex agent as long as you make sure to tighten the bolts at the specified "tape time" of the clear which is 5-6 hours for 2002 with medium hardener for instance.

Hope this helps.

CharlesPeoples

Should have painted with the mounting bolts tight ,or opted for the glass nose .Now you are doing that job twice ,what did it cost you in the long run.
PPG has a great system for that application,if done right you can prime, paint ,clearcoat ,and back on the car same day.Seen it done at PPG cert. school