Hood bonding problem

[Chaparral2f]

I can't get the urethane front fascia to bond (without cracking) to the front of the hood on on the tilt front end of my z31. If anyone has any advise on how to keep it from cracking, it would be greatly  appreciated.

[ihavearustedz]

What's the underside look like? Your hood may need some reinforcing. To me it looks like it's cracking from opening and closing that hood. It's pivot point is causing that sorta crack

[Chaparral2f]

Thanks for the reply. Here is what the underside looks like.

[JMortensen]

After seeing the frame underneath, my guess is that the two materials expand and contract at different rates, and it may be impossible to paint them together without it cracking. How about having a seam there and treating as two separate body panels? Other suggestion is take a mold off of it and do the whole hood in FG. 

[LLave]

Cool color. How about flex additive like they use for modern bumpers?

[ihavearustedz]

After looking at those photos, I'm going to agree with Jmortensen

[Chaparral2f]

  Thank you all for the help. I really appreciate the help.  The suggestion of building the whole thing from fiberglass has occurred to me, but doing it myself is way out of my skill range. and several years ago when I considered having one done the cost was much more than I could afford.

[Miles]

The following seems to apply here:

 

• The way that the  hood and the urethane front fascia interface has created a hinge point.
• Also, once the car is on the road, vibration will cause the bondo and paint to crack along the seem.

Possible solutions include:

 

• Stiffen the joint where the urethane front fascia and the hood  interface by adding a doubler plate under the seem and
• Modify  the support structure for the urethane front fascia such that hood and the urethane part move as a unit without flexing.
• Or leave as two separate parts with a seem showing as  JMORTENSON suggested above.

Best solution: Leave the seem showing

 

[cgsheen]

Vibration is not your only enemy here.  Even IF you could stiffen the attachment of the urethane to the hood, as to limit it's mechanical motion, there are still the laws of thermodynamics to content with.  Plastics and metals expand and contract at highly different rates with changes in temperature.  Even if you used the new flexible body bonding agents (which are amazing) and secured that interface well, you're still going to have the two disparate materials moving differently with temperature change.  Plastics are very thermally active - sheet metal not as much.  It will always eventually result in a crack between the two dissimilar materials.  I'm with JMORT and Miles - leave an exposed seam.

Edited December 7, 2018 by cgsheen

[JMortensen]

If you can fab up that tilt frame and the fender liners and all that, you could make a mold and do a FG version. It's tedious but not that difficult.