I'm finally to the point that I need to repair some fiberglass cracking on my '80. I've never done fiberglass repair so I've been watching endless YouTube videos. I'd still like to get some thoughts from those of you that have experience with this. I have some stress cracks in the front fender and front of the car where the bumper attaches. I also have some minor cracking in the rear, where the rear bumper attaches. I'll probably tackle the front fender stress cracks first, as they seem like they'd be the most straightforward to get started on. See the pics.

Front fender

So here are my thoughts, based on what I've learned.
-I've verified the cracks go all the way though the fender. So with that said, I believe I need to open up the crack with a grinder, or burr tool. I don't know how big to open it up, 1/4 inch? Then I need to apply fiberglass matting to the back side of the fender for sure. My question is on the front side. Do I apply new glass there as well. And then top it with Vette Panel Adhesive? I've also seen people just glass the back side of a crack, and then apply the Panel Adhesive only to the front side. Anyone with experience have ideas? I figure if I can get these cracks worked out, I'll be able to tackle the worse front cracks (see pics).



Finally on the back, there is some very minor cracking starting to show up where the bumper attaches. What's the easiest way to fix these? I don't believe they go all the way though. Do I just grind them with a burr tool and then fill them with the Vette Panel Adhesive again? Or do i burr them and apply more glass and then adhesive?

 

The beauty of fiberglass repairs is if you make a mistake, you just layup some more and resume so don’t get too concerned about how much grinding/sanding you do in the prep. Common sense goes a long way.

Be sure to use an epoxy based resin. While polyester resins were used from 53 until 72, GM began using epoxies in 73 so a polyester repair will potentially detach from the substrates.

Your repair layups should span the cracks by at least 1 1/2”. The strength of the repairs comes from the glass fibers, not the resin. While you may be tempted to blend the layers inside the fenders, those extra layers are what provides the strength. On the topside, taper blend the original glass back about an inch on other side. This allows the repair layup to span the crack, again providing strength.

If you grind/sand down the repairs properly, you can use just a skim coat of filler to achieve a perfect surface contour. Use a high solids activated primer (like Morton Eliminator) to allow for the final detailing before topcoating.

I’ve never used anything but new layup fiberglass in repairs. That cracked seam filler needs to be ground down the entire length of the seam and replaced with fiberglass. It’s a lot of work, yes, but if you do that, you’ll never have another similar crack. If you schlock on some adhesive locally, you’ll see new cracks in short order.

Be sure to wear a respirator (not just a mask) when doing body work. Protect your lungs!

 

Thank you very much for taking the time to respond to my post. While I've been dreading starting this for fear of messing it up, your post gives me confidence that in the end, even if it takes a couple tries, I should be able to get a good repair. Body work is definitely my achilles heal. I'm a mechanical guy through and though. However, when I get body shops telling me it'll be 25-30k to do the body repair, prep, and paint my car... that pretty much means I have to figure it out myself.

I didn't quite understand your statement: "While you may be tempted to blend the layers inside the fenders, those extra layers are what provides the strength. On the topside, taper blend the original glass back about an inch on other side." I don't quite understand what you mean. I'm assuming I do lay new glass on both sides of the fender. Are you saying that instead of just grinding the crack slightly larger and glassing over it, I actually attempt to taper the fiberglass to create a dish shape? Maybe I missed your point.

 

Your assumption about laying glass on both sides of the fender is correct. You won’t be concerned about the esthetics about the layers on the underside because no one will see those but whatever you apply on the sunny side will need to be ground down to the original contours. If you don’t taper the original panel to a feather edge (doesn’t have to be perfect) you won’t have a wide enough “trough” to allow the new glass to remain in after you’ve blended it to the original contour. In other words, a pencil wide “v” will result in a pencil wide repair which will be susceptible to cracking because it will be mostly resin.

What you could do is glass up only the inner side, let harden, then grind down the outside and layup. That will give you some additional confidence as you taper the outer side.

 

This is in my future and you sound like me so I will be watching and hopefully learning. Please post lots of picyures on your thread!

 

Agree good advice above.

The way I fix this is the following:

One thing I would add though is that since that fender lip is a high stress area, clean and scuff/clean the inside of the fender well and make the patch maybe 4 to 6" long centered on the break. That extra length will help prevent recracking again. Also, since the inside patch would not be seen, I would use fiberglass cloth which is stronger than the mat and about 4 layers laid in one by one saturated with resin. When you lay in the last layer, put a double layer of blue 3M painters tape over the mess and mash it down. This will help get the air bubbles out and give you a nice finish when hardened. On the inside, paint it semi black or flat black and it will be hard to notice unless you are specifically looking. Evercoat VPA for filler on the outside then finish

As above, googles and rubber gloves are good to wear.

Hope this helps.

Oh, forgot I would grind the crack completely out with a dremel tool with a clean gap of 1/16" or so.

 

The first time I got into fiberglass was back in 1974 when I decided to do some mild customization of my 69. Once I had stripped all the paint, I found out that the car had received a center wheels forward surround replacement and the factory L-88 hood I had acquired did not fit worth a flip. At that point, I realized I was in over my head so sought out someone local who could help. Turned out to be one of the luckiest breaks in my life for me because, to make a long story short, he taught me all I know about bodywork on Corvettes (C1, C2, and C3s as C4s were 10 years down the road at that time!) and custom painting.

As is so often the case, upon further inspection, found out the frame had been tweaked in the collision but was never corrected. Instead, whoever the Bubba was, worked around the issue by compensating in several areas of the repairs. I cut out the inner fenders, then took the car to a frame specialist and restored to OEM geometry. Without tramming the frame, would have never known. Set up the new inners, set the new hood, and glassed everything up. Perfecto.

One of the greatest lesson I learned working with him was that you want to think the repair through before doing anything. That starts with a fully informed assessment of what you have to start with, a clear vision of what you want in the end, and a well thought out plan of how to get there. Missing any one of those 3 will expose you to disappointment. The expression “look before you leap” has been around for nearly 800 years and is certainly applicable today. In that regard, bodywork and paint is no different than wrenching.

Your sticker shock on the quote for paint is not unexpected. Application of the paint is a small part, time-wise, in the painting process. Preparation for paint is where the bulk of the manhours are involved, with another time-consumer in the post-paint polishing and detailing. If you are willing and have the time, you can do all the prep and final detailing, saving you thousands. You certainly can apply the paint but you need to fully understand what is involved (including the health risks) and it may be best to sub that out to a trusted body shop painter.

Here at the forum, we’re here to help. You will find lots of opinions (which is actually a good thing because in those is where you learn) but in the end, you need to decide, based on facts, on which path you want to pursue. Good luck and keep posting, especially pictures!

 

You pretty much need to remove and replace the glass that is cracked. Grind the back side out and lay up 3 layers of mat. Now grind front side out and lay up until flush. Then bodywork it back flat. By flat I mean matching the contour it is now part of. It is difficult to fair in glass. Better to leave it a bit low and use vpa or other acceptable filler. Filler is softer than the original glass. So you are less likely to sand the original glass away as you are trying to bring the filled repair back to the original contour.

 

you can definitely pull this off. I was in the same spot months ago and definitely freaked out about the task at hand. Fast forward to today and I have done some major fiberglass work now... replacing panels, etc. The finish work for me still lies ahead, but one task at a time. As others have said, you want to glass on both sides of the repair area... a full thickness repair will be stronger than just one side.

 

I also use epoxy for all repairs, mold making and forming new parts. If you decide to use mat vs. cloth beware. Epoxy will not dissolve the binders in standard mat meaning you will not get it to lie down and contour to the surface. You will become frustrated, have lots of air bubbles and end up defeated. You can buy mat with binders compatible with epoxy, but you need to look carefully. For a repair like this one, I would use multiple layers of a light weight cloth, laying each layer 45 degrees to the previous. If you follow the good advice above on grinding and feathering, starting with the back side first, not using too much resin, and spreading the patch a few inches on each side of the crack, you will end up with a nice, permanent fix. Good luck and post up pictures.

 

You might want to hold your horses just a little on this: You are assuming your car is Fiberglass ("FRP"). It's not, and you can't use "fiberglass" to correctly repair it. Most panels on the 1980 are SMC ("Sheet Molded Compound"), and require use of products specifically intended for SMC repair. You need to educate yourself a little about the SMC-specific materals and processes before you cobble the thing up and make a mess out of your SMC panels.

Lars

 

dyanisis2 - if you navigate to the General Corvette Topics forum, then the Paint and Body subforum, and search the forum on SMC, there are numerous posts that provide a wealth of information (and opinions).

“Fiberglass” is the term most associated with the Corvette body but technically speaking, the original term for the material was FRP - Fiberglass Reinforced Plastic - developed by the Molded Fiberglass Glass company and Owens Corning. GM began introducing SMC (Sheet Molding Compound) in some panels in 71 but had transitioned over to SMC with the 73 model year. SMC panels do contain fiberglass, but their construction techniques require the addition of mold release agents and different resins (thermosetting). It is these resins and mold release agents that make polyester resin repairs incompatible.

You will find many instances where epoxy resins are not recommended because they don’t dissolve the binders in the fiberglass (repair) mat. Owens Corning used to sell a mat that was compatible with epoxy but that has been discontinued. You will read that many, many people use epoxies and swear by them. Using a vinyl ester resin is a good alternative but whatever choice you make, talk directly to to the resin manufacturer before making your selection.

 

I'm not sure what I said that implied I thought my car was FRP, unless it was just terminology. I do know it's SMC, and I have a cart of the products I intended to use in the pic below. I've been reading the body/paint part of the forum for quite a while. Unless I'm mistaken, it appears that from a repair perspective they aren't that different. I'd grind out the cracked area, knowing the SMC tends to crack laterally vs straight through, lay multiple layers of fiberglass mat, then top with filler and sand to body shape. I didn't think I was completely off the mark. Is there something obvious I'm missing?

 

I don’t think you are missing anything but often semantics comes into play to a greater extent when you are dealing with left brain dominant individuals (as in the majority of those who post on this forum). For example, your comment “…from a repair perspective they aren’t that different.” To some, they would think that is incorrect because, as you have found out, FRP requires the use of a resin that is much different than SMC. To others, that statement would be viewed as correct because of the steps you outlined, save for the use of a different resin, would be the same.

You are on the right track. Post some pics as you progress!

 

Thanks for your message
I looked at your photos
One you need strength front and rear
Here is what I do and my process
You need to V groove the crack all the way to the end - make sure to find its stopping point
Then bevel the V groove - meaning sand it back - you dont want a hard edge or a 90 degree edge
This is the edge you can see later down the road ( its called repair mapping )
You want alot of surface area for the material to grab
Once I V grooved it and bevel it - I would back tape the crack with Gorilla Tape then fill the front with VPA
Let that harden
Then cut some mat and replace the Gorilla tape with a couple layers of mat
If you stagger your mat - you can sand the back side for a seamless repair
Sand the front smooth - top with body filler - I use Rage Ultra
The biggest trick to cracks - it one, getting to the end of the crack and deep enough
2 - V groove and bevel
If you do that - your good to go
Once you finish your body work
Polyester prime - block sand
Urethane prime - block sand - (may need 2 primes - depends on the bodywork )
Sealer
Paint

 

Well I started working the first fender crack today. I noticed very quickly as I started V'ing it out that it had moved laterally within the glass a bit as well. I had to open it more that I expected to get to solid fiberglass. I've dished it out a bit, but I'm kind of guessing on how much to dish out. Three layers of glass on top seems like it might be high, but I don't know how much the glass with shrink down once it's saturated in resin. I've cut three progressively larger strips of glass to apply on the top. Is it best to use a single piece to wrap around the fender, as shown in the pic? Will it cause problems going from a nice round shape to a sudden vertical as it wraps to the front side of the fender?

 

if you would like to use glass
fill the crack with thin strips the length and width of the crack
progressively make them wider to fit the crack
back tape it first
You will prob need 3 strips maybe four.
I also use chopped glass in resin and make a mixture to fill the crack
or fill the crack with VPA

 

glass is hard. hard to sand to contour. and as hard as the glass the car is made of. you will need to use a sanding disk on a 4=1/2 angle grinder to get the new glass a bit lower than the glass it is patching. VPA or other fillers are softer than the glass and will coutour sand to the correct shape more easily and reduce-not eliminate-the amount you are sanding the car away next to the intended repair area.

 

As I mentioned in my earlier post #4, start with the underside first. Rough up the area with some 80 grit (by hand), then thoroughly clean with acetone. Pay particular attention to the area of the lip to ensure good adhesion.

You don’t need more than 2 layers. The first layer width should be as you have in the pictures with the second layer approximately twice as wide.

Follow the instructions for adding the activator to the resin exactly. Too little activator and the cure will be slow or, worse, never get to full. Too much and your cure time will be too short. Lay out the patch pieces on a smooth surface covered by aluminum foil. Apply the resin so the patch is saturated, but not so much that you have resin dripping all over the place. The strength of the patch is in the fiberglass, not the resin. Work any air bubbles out of the patch with your fingers.

Not sure of the environment that you are working in. If a heated garage, great but if not, you may want to pickup an inexpensive heat lamp. Don’t put the lamp too close to the work area. A couple feet away at least.

Once you have the backside patch cured, you can feather sand the outside so you have about 2” “trough”. 2 layers is all you need.

Once cured, get a couple pieces of PVC pipe of 2” to 3” diameter and wrap a sheet of 80 grit paper and hand sand to contour. Use a sanding block to achieve a straight edge on the lip. Take your time, sand a bit, feel the contours, repeat as necessary. You want to get the contours as close to original as possible so the only reason you’ll apply a filler is to fill the small air pocket holes that will be exposed while sanding.

Piece of cake!

 

What you posted for cut and size of patch doesn't look to be enough. Taper the original back at least 2-3" per side (so almost nothing left at crack tapering to nothing removed 2-3" away) and then use that thin piece first then a bigger and bigger piece as you layer it up. With the taper, the original area gets thicker as you go out so you need more smaller layers at first and fewer bigger layers as you build it up. That also lets you tie the fibers in each layer into the original instead of having a bunch of new piled on top of old with only 1 piece of new matting actually in contact with the old.