Fiberglass Experts

Wbulk · #1 (**permalink**) 02-22-2010, 08:58 AM

I am planning my next project and have been evaluating various fiberglass processes. Are there any experts out there who know what to look for to determine good quality vs. not so good, hand laid vs. chopper gun. I have noticed that there are various layers of material with some products. What do each of these do, and is there minimum thicknesses that a quality job should have for each layered material.

Wayne

DanEC · #2 (**permalink**) 02-23-2010, 05:40 AM

I'm not an expert but typically a hand laid up body is considered structurally stronger than a chopper gun body - all other things being equal. Then it gets into the resin they use, how thick they lay the fiberglass, the experience and technique of the fiberglass workers, etc. Some of the better manufacturer's will advertise they use a higher quality, high temperature - low shrinkage resin. I've been hearing lately that the SMC resin is far superior to the old polyester resin on fiberglass mat but I haven't seen that being advertised that I can recall. They typically use fiberglass mat which is the random strand mat since it doesn't imprint on to the surface like the fiberglass cloth that looks like cloth. Coremat is a thick mat material (say 3/16 inch thick or so) that is sometimes applied on the underside of fender tops to prevent stone and rock damage if kicked up. Thickness of fiberglass can be an indicatior of quality but also adds weight. I would not want to see anything less than 3/16 inch thick in general. Some areas go up to 1/4 inch for strength. If you see any areas in the 1/8 inch range that is getting pretty thin. All manufactuer's that I'm aware of use gelcoat in the mold before applying fiberglass and the thickness of gelcoat is an indicator of quality. It needs to be thick enough that you can rough sand the body for glazing and filler and leave plenty of gelcoat thickness to keep the fiberglass sealed and stable. And then the molds, or quality of the molds is a huge issue. The straightness of the molds and their setup and preparation for use have a huge impact on quality. Good molds and mold preparation should result in flush panels and very thin mold separation lines in the finished body. I'm working on an ERA and a couple of things I appreciate about their body are the thickened edges around the cockpit so I can roll them to mimick the original. Also, they make extensive use of interior fiberglass panels, or double panel construction for attaching hinges, hangers, and aluminum panels too. They sandwich metal plates in between the door inner and outer skin to carry the hinge and latch loads. They also roll under the rocker panel area similar to the originals. About all I can think of for now.

rustyBob · #3 (**permalink**) 02-23-2010, 09:26 AM

I agree with Dan...........
mine is a hand laid body and a friend of mine has a chopper gun body on his
car and they where painted by the same body man and he said it took him
twice as long to do my friends car because of all the air pockets in his body.
The sad part of this story is both bodies where made by the same company
they had changed from hand laid to chopper gun bodies.......

DanEC · #4 (**permalink**) 02-23-2010, 09:51 AM

I'm sure there are some good chopper gun bodies out there but the process of handlaying a body up requires that they roll in just about each layer of fiberglass mat resin to compact it. It's the rolling in of the mat and resin that so greatly adds to the density and integrity of the fiberglass panel. I don't know how most of the chopper gun places do their bodies but I think they rely mostly on the impact of the resin and fiber in the mold to compact it. They may roll it at some time during the shooting - not sure. They probably do at the end to seal the panel down.

strictlypersonl · #5 (**permalink**) 02-23-2010, 11:15 AM

Unless the chopper gun is spraying in waaaay too much resin, chopping requires rolling the fibers into the wet surface. Glass fibers don't automatically lay flat when they come out of the chopper.

DanEC · #6 (**permalink**) 02-23-2010, 12:14 PM

That's a good point. I remember seeing some 50s photos of GM laying up Corvette bodies with a chopper gun process - but after the fiberglass and resin was blown into a mold the panel was then transferred into a big press machine with top and bottom dies and the fiberglass compacted and cured. Called match molded or press molded panels which is what early Corvettes used.

strictlypersonl · #7 (**permalink**) 02-23-2010, 03:36 PM

Quote:

Originally Posted by DanEC

That's a good point. I remember seeing some 50s photos of GM laying up Corvette bodies with a chopper gun process - but after the fiberglass and resin was blown into a mold the panel was then transferred into a big press machine with top and bottom dies and the fiberglass compacted and cured. Called match molded or press molded panels which is what early Corvettes used.

After that process, I think that GM went to a system using Sheet Molding Compound (SMC) (a semi-cured glass-reinforced resin) which was placed in a matched male and female dies, compressed and heated to finalize the cure.

Many later "fancy" cars used Resin Transfer Molding (invented by Lotus) where the molds were much lighter and not necessarily steel. Dry reinforcing material was laid between male and female molds and the resin was injected and/or vacuum-drawn from one end of the mold to the other, saturating the dry material. Sometimes the mix was heat-cured, sometimes with catalyst only.

That concludes today's lesson on composite body construction.

Wbulk · #8 (**permalink**) 02-24-2010, 09:19 AM

So what is that white materal I have seen in the middle of the layers? I have seen gelcoat, a layer of glass, white material, and a layer of glass.

strictlypersonl · #9 (**permalink**) 02-24-2010, 09:25 AM

Quote:

Originally Posted by Wbulk

So what is that white materal I have seen in the middle of the layers? I have seen gelcoat, a layer of glass, white material, and a layer of glass.

Could it be straw colored, not white? (Kevlar)

Wbulk · #10 (**permalink**) 02-24-2010, 09:31 AM

It seems to be softer than glass.

If you sand through the gel coat is that a problem. Will not several layers of primer sealer be OK.

DanEC · #11 (**permalink**) 02-24-2010, 03:42 PM

No idea on the white material unless they put a layer of fiberglass cloth sandwhiched in between the fiberglass mat. Fiberglass cloth is very strong - just not good for finished surfaces.

It's better to not go through the gelcoat than it is to go through it - but it's not fatal. All fiberglass bodies need quite a bit of blocking work and having to take down a high area, through the gelcoat is common. Epoxy primer will seal it off before paint.

strictlypersonl · #12 (**permalink**) 02-24-2010, 04:17 PM

Maybe the white material is Coremat (which will look a little like felt).

It's OK if you sand through the gelcoat as long as you put a layer of polyester or epoxy filler over all the exposed fibers.

Wbulk · #13 (**permalink**) 02-24-2010, 06:51 PM

I bet you that's what it is. I could only see the ends.

Thanks.
Wayne

Larry Spillman · #14 (**permalink**) 03-08-2010, 03:42 PM

The advantage to handlaid is less likely to stress crack but a lot of fiberglass engineering has happened in the last 55 years mostly in boat building so a good new glass job should be equal too of better that a 55 year old job.But this America 2010 and its becoming a crap shoot out there Good Luck.