Lotus Elan

Epoxy vs. Polyester for Fiberglass Repair

PostPost by: deadline » Thu Mar 28, 2019 9:49 pm

I'm the new owner of a pile of parts that if I can get them repaired and back together might someday be an S3.

Ahead of me is a good deal of fiberglass work. Some of it quite structural in nature. In the past, for small cracks and dings, I have always use Polyester resin. Given the nature of these repairs I sought the advice of some "experts" at the local air field where fiberglass planes are very common and their repair a frequent event. I have been directed to use Epoxy over Polyester resin. The reason cited was "it's 25% stronger". Not being an expert I thought I would ask here if there was a consensus.

So which is better, Epoxy or Polyester, for Fiberglass Repairs on a Lotus from 1966 made with whatever was used then?

for reference: https://www.epoxyworks.com/index.php/we ... at-repair/
Yes, I know it's a boat link but West System Epoxy is what the plane guys use too.
https://www.aircraftspruce.com/catalog/ ... tepoxy.php
Last edited by deadline on Fri Mar 29, 2019 12:22 am, edited 1 time in total.
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PostPost by: Grizzly » Thu Mar 28, 2019 11:34 pm

In my opinion it depends what your doing with it, if you are repairing a crack in a panel Polyester adheres to it's self as well if not better than Epoxy would stick to polyester so there is no point in paying the premium.

Epoxy works best as almost a glue, for example if you are glassing two moldings together or fitting bobbins or strengthening rods etc for this it's much better than Polyester.

Bear in mind Polyester won't stick very well to epoxy but epoxy will stick to Polyester very well so if you do use Epoxy you need to stay with it.
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PostPost by: Bud English » Thu Mar 28, 2019 11:51 pm

I'm certainly no expert, but I used polyester resin and glass for all my repairs of original fiberglass damage and saved the West Systems epoxy for a final barrier coat over everything before going into paint.

I also used epoxy for interior panel repairs, the center console, my replacement dash pad (EPS foam) and any other polystyrene based plastic parts. Epoxy won't melt polystyrene and polyester resin will.
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PostPost by: deadline » Fri Mar 29, 2019 12:17 am

Bud English wrote:Epoxy won't melt polystyrene and polyester resin will.


Does that imply the the original bodies were made with polystyrene and by using polyester resin you could get more than just a surface bond?

I'm sure there is some chemistry going on here. Does anyone know the rule of thumb that applies?
Polyester over polystyrene or Epoxy over polystyrene but not Epoxy over Polyester or something like that?
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PostPost by: Lotus14S2 » Fri Mar 29, 2019 12:32 am

Speaking from the point of view of an engineer, who for a period of time, worked on designs for aerospace in various plastics.
Now having said that, I’m sure there will be a bunch people to disagree with me, but here goes anyway:
Most real aircraft plastic construction uses “prepreg”, or a woven sheet like material which has plastic filler that is cured by heat. Most of these are an epoxy type material, although polyester, as well as other plastics is used. Even repair patches and panels, are similar, and use heat to cure. I designed tooling to make parts for the B-1 bomber, and this was the method used. We had autoclaves, big enough to fit huge sections of the plane.
The material that has the strength in resin and glass, or other fibers, is the fiber, not the plastic. If properly done, you have a minimum of the plastic resin, and as much fiber as you can manage. When doing this by hand, without vacuum bagging, for example, you tend to get more plastic than is the ideal. But that is the way most fiberglass cars, boats, or bodies are made, and they all seem to work OK.
The original Corvettes were made this way, but the later ones used steel tools, and the material was forced under pressure into the molds, and the parts look completely different than a Lotus. The Corvette parts sort of look like plastic kitchenware; if you’re familiar with old Melmac plates.
The point is, if properly done, polyester resin is just fine for most glass fiber construction, and the need for epoxy, is not necessary, and is considerably more expensive.
If you think its cool then use it, but polyester is much easier, and more forgiving to use
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PostPost by: Bud English » Fri Mar 29, 2019 2:43 am

deadline wrote:Does that imply the the original bodies were made with polystyrene and by using polyester resin you could get more than just a surface bond?

No, not at all. I was talking about the white thermoformed interior parts. The center console, under dash panels and dash trim panels on my '70 +2S are all white sheet styrene under the vinyl upholstery. The epoxy bonds to that well for making repairs, but polyester resin just dissolves it.

The body parts are polyester resin and chopped fiberglass. Repairs are no problem with either epoxy or polyester resin. But, like Grizzly, I prefer to use polyester resin for body repairs.

Sorry for the confusion.
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PostPost by: StressCraxx » Fri Mar 29, 2019 4:09 am

Lotus14S2 wrote:Speaking from the point of view of an engineer, who for a period of time, worked on designs for aerospace in various plastics.
Now having said that, I’m sure there will be a bunch people to disagree with me, but here goes anyway:
Most real aircraft plastic construction uses “prepreg”, or a woven sheet like material which has plastic filler that is cured by heat. Most of these are an epoxy type material, although polyester, as well as other plastics is used. Even repair patches and panels, are similar, and use heat to cure. I designed tooling to make parts for the B-1 bomber, and this was the method used. We had autoclaves, big enough to fit huge sections of the plane.
The material that has the strength in resin and glass, or other fibers, is the fiber, not the plastic. If properly done, you have a minimum of the plastic resin, and as much fiber as you can manage. When doing this by hand, without vacuum bagging, for example, you tend to get more plastic than is the ideal. But that is the way most fiberglass cars, boats, or bodies are made, and they all seem to work OK.
The original Corvettes were made this way, but the later ones used steel tools, and the material was forced under pressure into the molds, and the parts look completely different than a Lotus. The Corvette parts sort of look like plastic kitchenware; if you’re familiar with old Melmac plates.
The point is, if properly done, polyester resin is just fine for most glass fiber construction, and the need for epoxy, is not necessary, and is considerably more expensive.
If you think its cool then use it, but polyester is much easier, and more forgiving to use


Have you ever used vinyl ester resin? I think it is easier to use than polyester and much less expensive than epoxy. It's more resistant to blistering and more water resistant.

Regards,
Dan
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PostPost by: Bombay Racing Green » Fri Mar 29, 2019 6:28 am

Hi,

I have found this channel quite interesting: https://www.youtube.com/user/boatworkstoday. I hope it helps.

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PostPost by: RichC » Fri Mar 29, 2019 6:32 am

@deadline, I'd say keep it simple and original... polyester
there's always the law of unforeseen consequences to contend with, as has been pointed out below( or above).
One extra note, which i don't think has been mentioned, is that if you emulate the the original structure and try not to 'add in' extra rigidity , i'd suggest you're less likely to incur stress cracks from the rough and tumble of daily use . 8)
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PostPost by: Concrete-crusher » Fri Mar 29, 2019 7:38 am

I use epoxy resin and then polyester fillers. I'm just about to apply some tissue to epoxy today to deal with a couple of stress crass I have uncovered while sanding back body planels. One point to observe the chopped matting is different for both. Matting designed for polyester does not wet out in epoxy.

I have also started using peelply which prevents too much resin build up when finished

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PostPost by: Esprit2 » Fri Mar 29, 2019 6:45 pm

For composite repairs that have a cosmetic element to them (auto body work), it's best to use the same resin with which the part was originally made. If you just want the strongest, and don't care about cosmetics, then use a quality epoxy, like Proset.

Each resin has it's own color, even if just a light tint. Eventually, that color will telegraph through to the surface, and become visible in light colors... dark colors to a lesser degree. The Elite Type 14's body shell was molded out of polyester, but some of the add-on parts, like the doors, bonnet & boot lid were molded out of epoxy and at least one other resin. An older restoration in white, on display in the sun, will be a patchwork of panel colors. It's not a poor paint job, or poor prep, it's the different base resins from which the body panels were made. For a 50-50 paint job on a racecar... maybe you don't care. But if this is your pride and joy, and you want it to be nice going forward, stick with the body's original resin... polyester.

Plastics, including liquid resins, have a shrink rate. The vast majority of the shrink occurs very quickly after molding, but it continues at a very slow rate for a very long time. Depending upon the resin, it can be for months, or many years. Have you ever noticed a split in the rim of a vintage car's steering wheel? That's because the outer, over-molded plastic rim continued to shrink over time, while the inner steel core didn't move at all. The long term shrink put the rim in tension until it couldn't take it anymore, and it split. Then the crack continued to open up, and it's not uncommon to see a 1/4" gap. That's shrink.

The various liquid resins used for composite molding all have different shrink rates. If you use something like epoxy to make a large repair in the middle of a polyester panel, you can fill & sand to perfection such that no one will be able to see the repair. For now. But over time, the polyester panel and the epoxy repair will shrink at different rates, and eventually the perimeter of the repair will become visible through the paint. Not next year, or maybe the year after that, but eventually. So, are you doing all this work for immediate gratification, or do you want to be proud of the car at a concours 10-15 years from now. If you're just doing a quick Mary Kay job on the car to flip it, then maybe you don't care. It'll be the next guy's problem, so who cares... right?

Vinyl ester is close to polyester in many ways, but it does have a different color and different shrink rate. Epoxy is different by a larger margin.

For race car meatball surgery, epoxy is stronger, more heat resistant, and bonds to polyester better than polyester bonds to polyester. In many structural ways, it's better. And from 50 feet away at 50 mph (50-50 paint job), it will look pretty good, so who cares... right?

Similarly, for a long term cosmetic finish, use matte, not woven roving (cloth). No matter how well you fill, sand and finish, eventually the weave pattern will telegraph through to the surface. Even a layer of cloth 3/32" to 1/8" under matte will eventually show through. Use matte for a cosmetic restoration, and save the cloth for the race car.

It's all about time. Are you working for the short term, or looking down the road?

Regards,
Tim Engel
Last edited by Esprit2 on Sat Mar 30, 2019 11:18 pm, edited 1 time in total.
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PostPost by: LaikaTheDog » Fri Mar 29, 2019 7:23 pm

I have done whole bodies like bud. Poly for repairs and cracks, then a full gel coat in epoxy to even it all out, remove bubbles etc., then painted in 2pack.
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PostPost by: Gordon Sauer » Sat Mar 30, 2019 4:57 am

I guess I like the idea of doing it the way it was and knowing that that worked out so when I was redoing my +2 I did it in polyester always and yet used cloth so it would be stronger and I’ve had no stress cracks return after more than 10 years now but additionally the Elans were all gel coated and I did re-gelcoat all the cloth and perhaps that’s why none of my cloth has ever shown through the paint. Gordon Sauer
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PostPost by: RichC » Sat Mar 30, 2019 7:23 am

@Tim Engel .. nice response . thank you for sharing that 8)
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PostPost by: denicholls2 » Mon Apr 01, 2019 1:39 pm

LaikaTheDog wrote:I have done whole bodies like bud. Poly for repairs and cracks, then a full gel coat in epoxy to even it all out, remove bubbles etc., then painted in 2pack.


This is pretty much in line with my thinking. Poly cures in 15 minutes, Epoxy more like 15 hours. Do you want to wait 60 times as long for your repair to set?

I would not say the heat issues with poly are about chemistry between substances. The catalyst properly mixed simply reaches a much higher temperature than epoxy, high enough to melt other plastics near it.

While both are hard to sand, epoxy is harder. Getting the shell to a good surface after repairs will be easier with poly.

Over veil, the slow curing of epoxy on a well-prepped surface will pay off in a smoother final shell. Because it won't be curing while you apply it, you can build over the weave to the right amount and it will flatten as it cures, something poly won't do well. This means you can add thin coats until the fiber is properly covered and not leave a lot of surfacing work. You can also roll epoxy; don't even think about trying that with the short pot life of poly.
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