stress cracked fiberglass repair advice please

M

marty68

Promoted Users
i have been helping a friend do his 79 firebird. i only have hood and bumpers left to do. the hood is an aftermarket fiberglass hood. it has a coupe spots of cracks where the "gel coat? seems to be cracked.
can i feather
out with 180 and use 3-4 coats of epoxy to "bury them"
asnd out epoxy and a bit of filler re epoxy?
my plan for the rest of the hood is block with 80 to see how bad it is then a couple coats of epoxy block again to determine if i need to use polyester primer or if epoxy and or 2k will be good enough. i HATE seeing fiberglass parts that are fiberglass wavy. attached is where the car is today, and pictures of the hood
thank you
Marty
 

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The gel coat cracking in the manner it is in those pics is indicative of it being hit by something there or flexed. Possibly the hood being shut on something left in the engine bay. Likely the fiberglass is cracked under that. Sand the gel coat off in those areas and inspect.
There are several ways to repair cracks like that. All involve v grooving the crack. If you just sand it and feather out the cracks will likely return. I have my doubts epoxy alone will be enough. I would want to v-groove and repair if it were me.
 
It does look like it may be impact damage from the inside to me as well. Is that hood hand laid glass or SMC? Is the underside smooth or have a fiberglass look to it?
In addition, SMC products will work on glass as well, not the other way around. If you choose to grind out and re-glass, use epoxy resin. Grind out with 36 grit, clean with acetone, lay out your glass and epoxy resin. Give 24 hrs, feather out, skim with P17, block and epoxy prime.
 
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one definately WAS an impact probably BOTH. YES it is fiberglass. underside can see glass strands. it is fairly thin 3/16" roughly

what is my plan? i havent done any fiberglass work since the 80s.
so can i grind it mostly down apply epoxy resin cloth keeping it lower than hood. or do i cut a piece out and create a "patch"
is there a thread on here explainig this..
what is "p17"
 
marty68 said:
one definately WAS an impact probably BOTH. YES it is fiberglass. underside can see glass strands. it is fairly thin 3/16" roughly

what is my plan? i havent done any fiberglass work since the 80s.
so can i grind it mostly down apply epoxy resin cloth keeping it lower than hood. or do i cut a piece out and create a "patch"
is there a thread on here explainig this..
what is "p17"
Click to expand...
If it is spiderwebbed or multiple cracks intersecting with one another the the best plan is to cut the section out and glass in that area.

P17 is a product for fiberglass by Adtech. It fills and bonds. Very good stuff.

Search the site in this manner using Google. site:spiuserforum.com fiberglass

You can use different/multiple keywords.
 
Great info. Not trying to hijack thread but I have a fiberglass sidecar and the tub has spider cracks in one area and minor crash damage covered by a big sticker in the front.
When you say cut out the spider area do you mean cut all the way through or just dig out the first couple of layers under gel coat?
 
Kawickrice said:
Great info. Not trying to hijack thread but I have a fiberglass sidecar and the tub has spider cracks in one area and minor crash damage covered by a big sticker in the front.
When you say cut out the spider area do you mean cut all the way through or just dig out the first couple of layers under gel coat?
Click to expand...
Cut it all out. Then grind the edges of the hole so that the edges are feathered out. Knife edge might be a better term. You want it feathered along the edges of the hole so that you have maximum strength when laying more fiberglass. Especially important if you can't put a supplemental patch on the underside because of cosmetic reasons.
 
Kawickrice said:
Great info. Not trying to hijack thread but I have a fiberglass sidecar and the tub has spider cracks in one area and minor crash damage covered by a big sticker in the front.
When you say cut out the spider area do you mean cut all the way through or just dig out the first couple of layers under gel coat?
Click to expand...
Long answer,
You remove what is needed to eliminate the damage. For example, look at the last picture of the hood. Say an object like a 9/16 socket or something was left in the engine bay and the hood slammed down. The initial impact is only 9/16, but the cracks and disband transferred through to the topside and is about 3” or so. If you grab a quarter and tap on it you will hear a dull thud for disbonded areas instead of a ringing sound. Grind out the damage and feather back edges. By this point you may have a good size hole, use foil tape as a backer on the inside then lay your layers of glass starting small and extending out each layer a little larger to build thickness back up.
Short answer,
Don’t just grab a cutoff wheel and cut a square.
IMG_0471.png
 
Chris_Hamilton said:
Cut it all out. Then grind the edges of the hole so that the edges are feathered out. Knife edge might be a better term. You want it feathered along the edges of the hole so that you have maximum strength when laying more fiberglass. Especially important if you can't put a supplemental patch on the underside because of cosmetic reasons.
Click to expand...
underside is ugly and will remain ugly. they will be.
cut/ ground out. i was just informed i need to "fill" hood pin holes at the rear anyway. the car is my friends, I am "guiding him" and doing the spraying
 
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