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Old 04-20-2008, 08:27 PM
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HELP cracks on fiberglass body!

EDIT_ gel coat is .5" thick not an 1" like i thought----

History on the project- roughly two and a half years a ago we ordered a fiber glass 35 Chevy coup body from Antique & Collectible Autos Inc. When the car arrived to are shop it Had no cracks.The body was left out side in the crate for round 2 weeks. When we went to test fit the body on the frame we noticed 2 cracks about 3 each long on the top of the body at this time we didnít think much of the cracks. A month later the car had around 30 small cracks on the body. I called Antique & Collectible Autos Inc and was given a number to the guy that makes the bodyís .He told me to email him photoís .Basically He wrote back saying the cracks need to be fixed.

A year later from when the body arrived we started the body work. Realizing that some of the cracks were only in the gel coat and some had gone all the way down to the glass. Also the gel coat was up to a 1Ē thick on spots where it has cracked. At that point we fixed all the cracks and did a matt red paint job on the car. Now one year later from the time we painted the body we now have around 20 NEW cracks that are on parts that were previously fine. Once again some of the cracks are only in the gel coat and some are all the way down to the glass

What could be causing all the new cracks? Iím at a point on this car where I want to give up

FYI= body is bolted down to the frame on 8 spots. Cracks are on the rear body by the trunk ,top of the roof by the doors and on the body line that go across the boors. seems like the cracks happen when the car sits in the sun

Note_ sorry not the best writer
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Last edited by Adamrmr; 04-21-2008 at 09:45 AM. Reason: wrong info
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Old 04-20-2008, 11:03 PM
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Bad resin possibly?

Shane
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Old 04-21-2008, 12:08 AM
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Quote:
Originally Posted by Adamrmr
H

Also the gel coat was up to a 1Ē thick on spots where it has cracked.
What could be causing all the new cracks? Iím at a point on this car where I want to give up


Gelcoat backfill is WAYYYYYY too thick. One whole inch????

Here is a lesson in proper use of gelcoat.

Gelcoat is the resin rich coating that is applied to the mold surface prior to laying up the actual fiberglass reinforcement.

It has several purposes...First, it is a shiney surface to give the part curb appeal. Second, it is barrier to prevent moisture from getting in the fibers, it also prevents UV rays from deteriorating the resin, and exposing the glass fibers, It can be either the color coat, in an as molded consumer ready product, such as a boat or bolt on fender flare, or it just a base coat to isolate the fibers from the surface, so they do not soak up primer and paint when you apply a finish...


In no event should the gelcoat be more than .020"-.025" thick...Ever.

If it is applied even .100" thick eventually it will crack, just as you describe, and short of keeping it inside forever, or stripping it off, it will continue to crack forever as it shrinks.
To fix it properly, the gelcoat needs to be stripped down very close to the glass, allowed to normalize for several days in the sun, the corners, belt reveal lines and eyebrows rebuilt with glass, or a reinforced filler and a filled polyester surfacing primer applied. Some like to use epoxy, either way, that thick, unreinforced mess needs to come off.

You could grind the cracked surfaces down about 1/8, and glass over the over the area with 3 layers of 1 1/2 oz matt, (not cloth)then do your bodywork and reprime, but that is a ton of work...

If that gelcoat is an inch thick, or unfilled / unreinforced resin under the gelcoat is that thick, if left that way, it will be neverending problem.

I know that some shops will fill any reveal lines or sharp detail lines such as the eyebrows over the door openings with gelcoat or pure resin, prior to layup, those always crack.


The reason that shops use resin in the corners is because 'glass won't make sharp bends, a bubble will appear as the glass springs back from the corner, and the outside edge will break through after the part is de-molded..



I'd send that body back, but the guy will probably not accept it, as it has been painted and it has been such a long time, although it is clearly a manufacturers flaw.


Who is the subcontractor who builds the body? He ought to be ashamed

I do know a little bit about fiberglass, I worked at Poli Form Fiberglass for 13 years, building fiberglass replica bodies and fenders. For pre filling sharp detail lines like described above,, we use a catalyzed reinforced filler, made from polyester resins and several different types of fillers as well as milled fibers.





Later, mikey
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Old 04-21-2008, 06:21 AM
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I totally agree with Mikey, the gel coat is way too think, and in addition I would suspect the body manufacturer did not use quality a quality resin.

Vince
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Old 04-21-2008, 06:46 AM
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Mikey is dead on. I have never seen gelcoat 1" thick.But like he said any gelcoat or resin that is not reinforced will continue to crack.Definitely a manufacturing defect.Please try to reveal the manufacturer so nobody else buys from them.Good luck.
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Old 04-21-2008, 07:36 AM
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I agree with above posts and would also add ....

Even gel-coat that is the accepted thickness will crack if the laminate is subject to stress or flexing. Remember most 'glass bodies don't have much strength and require stiffning such as metal framework, (esp. around doors, trunk, cowl). Make sure the body is not flexing and add stiffners if needed. This could be anything from welding additional framework to laminating ribs to the inside of the body.
Been working on 'glass boats, cars 20 years and never seen 1 inch thick gel-coat!

Remember anything can be fixed and most problems on 'glass happen in the first year or so when the curing/shrinkage process is still going on.
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Old 04-21-2008, 09:48 AM
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Going to make sure that the body is not flexing .Any tips on metal frame work for the inside?
thanks for the help
the car has outlaw front and rear fenders with not one crack ..I'm going to fix all the spots And try heating the body in the booth to see if anymore cracks
EDIT_ gel coat is .5" thick not an 1" like i thought----
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Old 04-21-2008, 11:07 AM
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Even a half inch is too much..when I was building boats we only shot about 10 mils (as I remember) of gel coat and we had to cut samples of the layup so it could be tested ..got the samples from cutouts from thru hull fittings and where we cut for portholes and such..

I make my inner frames from square tubing..I get my bender dies from JD squared so I can get the proper die with a phone call if I need something I do not have..Just bend the tube to fit the contours of the inner body as close as possible and then glass over and around them.I like doing that in the door post area as it gives me something solid to attach door hinges to......Another thing one can do is to get yourself some balsa core like we use in the boats and glass this to the inner side of the body as this will stiffen things up a whole lot..Adds weight but on a street rod should not matter that much..

Once that is all done then make the repairs to the gel coat..Use kitty hair or a mix of resin and chopped glass to build out the areas needing it and then I would just forget about putting any more gel coat on..just use a good epoxy primer and then paint this thing..

One of the major considerations in laminating is your resin to glass ratio..maximum glass content with just enough resin to glue the whole thing together is what we shoot for..Another thing before you start get a good manual on laminating glass and follow instructions..

Kinda long but I hope you get the drift of this..

Sam
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Old 04-21-2008, 12:26 PM
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add just abit more "food for thought" about where and how to brace the body....

when you do hit a pot hole or rough rr tracks that massive impact force on some parts of the body needs to be distributed over a large area as with onemoretimes balsa reinforcement example.....

some points like the door it's better to lock it down for no movement...

some points it's best to build in flex as with a rubber mounting which can also help with expansion and contraction due to heat....

do be thinking about if I reinforce it here...where will that pot hole impact stress be moved to and disappated....

it's gona take some careful planning because ...
the present (to much) gel coat flex factor doesn't match the laid up glas flex factor now...
so when you add pot holes stress to the equation....wow
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Old 04-21-2008, 01:48 PM
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Quote:
Originally Posted by OneMoreTime
Even a half inch is too much..when I was building boats we only shot about 10 mils (as I remember) of gel coat and we had to cut samples of the layup so it could be tested ..got the samples from cutouts from thru hull fittings and where we cut for portholes and such..

I make my inner frames from square tubing..I get my bender dies from JD squared so I can get the proper die with a phone call if I need something I do not have..Just bend the tube to fit the contours of the inner body as close as possible and then glass over and around them.I like doing that in the door post area as it gives me something solid to attach door hinges to......Another thing one can do is to get yourself some balsa core like we use in the boats and glass this to the inner side of the body as this will stiffen things up a whole lot..Adds weight but on a street rod should not matter that much..

Once that is all done then make the repairs to the gel coat..Use kitty hair or a mix of resin and chopped glass to build out the areas needing it and then I would just forget about putting any more gel coat on..just use a good epoxy primer and then paint this thing..

One of the major considerations in laminating is your resin to glass ratio..maximum glass content with just enough resin to glue the whole thing together is what we shoot for..Another thing before you start get a good manual on laminating glass and follow instructions..

Kinda long but I hope you get the drift of this..

Sam
thanks
what size of square tubing should i use? ]u guys think i can find a bender for under $300?also can round tubing be used and if so what size?

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Old 04-22-2008, 05:32 AM
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If I was doing it I would use square (about 1 inch) as it is easier to add webs and attach body/fasteners/tabs etc. This link might give you some ideas.
http://www.wescottsauto.com/body-steel/p-steel-1.htm

I would check flex at chassis (which might be causing your roof crack), before adding to body framework it might be easier to stiffen the chassis than the framework.
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Old 04-22-2008, 07:06 AM
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Adamrmr,

scrimshaw's link explains better (what I tried to write) about some parts do need to flex and expand and contract....do study that link
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Old 04-22-2008, 07:34 AM
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Quote:
Originally Posted by Adamrmr
thanks
what size of square tubing should i use? ]u guys think i can find a bender for under $300?also can round tubing be used and if so what size?
If I were going to use round tube I would go with 1 5/8 0.134 wall and make a roll cage in the car..but then again I have been upside down in one of these things and a cage and a proper 5 point harness is a good thing in my book..Just attach the body to the cage with some tabs welded to the cage..

JD Squared or Pro-tools have reasonable priced benders but the dies are what will get ya in the wallet...but then again for myself it was well worth the investment to be able to bend tube to the specs that I need..

Sam
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Old 04-22-2008, 08:07 AM
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One more time,I like what you said about the resin to glass ratio.Any non reinforced resin will crack.I have seen some people add a layer of resin to smooth out their glass work and that is wrong.
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Old 04-22-2008, 08:16 AM
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The cracks in the eyebrows and belt reveals are from excessive shrinkage caused by too thick of unreinforced material .

The normal body flex that occurs with hitting potholes does not cause cracking like that, Fiberglass made even moderatly inexpensive resin, in th eright ratio, and gelcoat in the right thicknesses are very flexible. (How many fiberglass fenders are reinforced with steel?)

Only when a localized stress is imposed on the laminate,, such as a load from a door hinge or a striker, does cracking like that occur.


The link to wescotts is a good one, and the methods they use to reinforce their bodies are time tested and are designed to spread the localized stress loads across the stressed fiberglass skin, through careful planning of attachment points and structure design.

The point that red65mustang is trying to make is that you don't want the steel to be tight against the fiberglass, as this in itself will create a hard stress point, and possibly cause a fracture in the gelcoat from the inside out, (just like the "star" you get from a rock hitting the underside of 'glas fender).

Here are some guidelines for reinforcing a glass body.

Start at the points where the body bolts to the frame, you can make footings that are bolted down to the frame and build your structure up from there. Utilize the strength in the chassis to reinforce the body.


Make plates that the hinges, strikers , latches etc can bolt through, these plates should be large enough to be welded to your tubing structure. The plates can be bedded in to the body skin, using some kind of bedding compound that is compatable with steel and fiberglass. Any body filler type material works, so does epoxy and there are many materials available from the fiberglass supply houses that will do the job.

Plan on putting steel around all openings and creating a steel "cage" that will tie all of the mounting points together.

Do not let the steel touch the inside of the glass skin in places where it will be noticed from the outside.
This will cause the "mirroring" that wescotts referrs to in that link.

Mirroring is also called "transfer", "telegraphing", or "print through", and is caused by something with a different coefficient of expansion and of a different hardness being attached to the inside of the skin in a secondary lamination with no clearance, and no space to allow for shrinkage..


Here is what happens. A panel is laid up. It cures, shrinking slightly as it gets hard. After about 12 hours at 70 degrees, although the shrinkage is very slight, it will continue to shrink for years.

Now a steel tube, wooden rib, or another peice of cured fiberglass is laid against the inside of the panel, and fiberglassed over to attach it.

The layers of glass used to tie it down, shrink themselves, pulling the new part tightly against the backside of the laid up panel. The panel gives a little bit, and protrudes slightly towards the outside. You now see the imprint of the steel, wood, whatever coming through. Put that car in the sun, and it will show up even more. as the glass panel moves around from expansion. Even steel that is just laying tightly against the inside of the body will print through, because the glass skin is still shrinking throughout it's lifetime. (I've seen print through occur after several years..after the car is painted and done.. )


This is why you do not attach the steel to panels that will be seen from the outside. use return flanges such as door jambs, driprails and windshield flanges to attach your steel to.

Use a somewhat resilient bedding compound, or even some coremat, foam or soft rubber to go between the steel and fiberglass to take up the shrinkage.

We used 3/4" square tube in most of our bodies, and never had a problem.

The best way to describe the theory of design is to say that you build a hoop structure, picking up attachment points of various loads, then attach it to the skin at the points that will best transfer the stress back down towards the frame. In effect, you create a truss using the stressed outer skin and the steel. You dont need too many attachment points to make a body strong, and keep it from cracking from load induced stresses.


Be sure to design some mounting points for your steering column, and seatbelts. Even closed cars can get the cowl moving around from the steering column being torqued around when driving.


I could go on about this for days.

There is a book called "how to build fiberglass kit cars."
http://www.amazon.com/Build-Fibergla.../dp/1884089100

Get yourself a copy, it is well worth it.


Hope this helps,

Mikey
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Last edited by powerrodsmike; 04-22-2008 at 08:47 AM.
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