Bonding fiberglass to steel

[CG]

Been a long time since I have thought about this. A million years ago or so I put a six pack scoop on one of my Mopars, looked great for a while, and then, blech. Ive been looking at some pics online of hood scoops I was wondering if technology has come along that has figured out how to bond fiberglass to steel without the seams eventually cracking?

[MP&C]

As long as you stay away from the bondo brand types of FG resin (clear), there are adhesives out there that will do the job. This hood was bonded to steel hood many decades ago. Car has all the flex and vibrations that go along with its blower motor with no issue..


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[Foot Stomper]

Yes, Lord Fusor 108B I believe will do the job.

[StingRay]

No matter how good you bond it the joint will show a ghost line if you try to blend it. The two materials expand and contract at different rates. Any kind of filler at the joint will inevitably crack. If you just want to bond it on with a hard edge and no blend then yes it can be done but you will need an adhesive that has some ability to absorb some the the difference in expansion rates. A good example is urethane like what holds the windshield in. Methacrylates work well for this too. They have a tenacious bond. You can't even make a steel to steel bond in the middle of panel and not have it show eventually.

[MARTINSR]

This is the thing though, these adhesives have changed! Let me put it this way, we just put a quarter on a $60k 2018 BMW at work and going by the BMW guidelines didn't use a single weld bonding even the C pillar splice with a backing we got from them and then putting "metal filled" bondo over the top of that seam!

I have to believe that bonding it with "structural" adhesive then putting a fiberglass reinforced filler over that to blend it out onto the metal hood creating a decent thickness like a 3/16" inch or so will keep the ghost lines away and it sure as hell isn't going to crack or something like that, I can't imagine! These adhesives of today are friggin amazing!

But on the same note, as suggested to fit that scoop perfectly, grinding and sanding the edge so it looks perfect all the way around then bonding it and leaving that seam would be the absolute best way.

I have done many tests over the years, this is a big one but I need to so it. I need to bond a piece of fiberglass to metal and bolt it to the frame under my daily driver and leave it there for months and see if there is a ghost line, hmmmmm, I like tests like that.

Brian

[StingRay]

Where I work we build 800 vehicles a year and for at least the last 20 years they've had fiberglass bonded to steel. Bonding is easy. We gave up a long time ago trying to have paint bridge the material joint. The paint gets a fault line where the two expand and contract differently. Same goes for any kind of filler. I guess if your climate is fairly constant then you might get away with it. Fiberglass has real glass! A windshield requires a very flexible bond material or the glass will crack. The adhesive needs to take up the difference and it needs thickness to do it. Without that thickness either the adhesive fails or one of the parts fails! Make a hard edge with a visible edge and paint up to that edge and no problem. The pieces can expand and contract independently. You can stick the two together........seamlessly good luck. On a hood with all of that heat you'll need the luck and then some.

Where I work I'm the adhesives guy. We use hundreds of thousands of dollars worth of different adhesives on a hundred million dollars worth of vehicles a year.

I wouldn't waste my time trying. Been there and done that.

[MARTINSR]

Originally Posted by StingRay

Where I work we build 800 vehicles a year and for at least the last 20 years they've had fiberglass bonded to steel. Bonding is easy. We gave up a long time ago trying to have paint bridge the material joint. The paint gets a fault line where the two expand and contract differently. Same goes for any kind of filler. I guess if your climate is fairly constant then you might get away with it. Fiberglass has real glass! A windshield requires a very flexible bond material or the glass will crack. The adhesive needs to take up the difference and it needs thickness to do it. Without that thickness either the adhesive fails or one of the parts fails! Make a hard edge with a visible edge and paint up to that edge and no problem. The pieces can expand and contract independently. You can stick the two together........seamlessly good luck. On a hood with all of that heat you'll need the luck and then some.

Where I work I'm the adhesives guy. We use hundreds of thousands of dollars worth of different adhesives on a hundred million dollars worth of vehicles a year.

I wouldn't waste my time trying. Been there and done that.

I would say that is some valuable advice.

Brian

[CG]

So basically nothings changed since I did my Mopar scoop back in the day except you can get it to stick better. Just forget about doing it seamlessly. If I want a scoop Im going to need a fiberglass hood ... Thanks for the insights!