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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Body - Exterior> Resin vs. Epoxy for fiberglass work?
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Topic Review (Newest First)
03-04-2010 12:20 PM
steve392 Cboy,

If you haven't already found these sites, check them out:

They have a lot of useful information regarding resin types, cloth types, etc. in their Learning Center link.

Another one is

They also have a learning center.

The last one, which I bought my material from when I made my trans tunnel (documented in my project journal) is this company:

They have good prices, and also a FAQ page.

There are big differences between an epoxy resin and a polyester resin. Epoxy resins can come in different viscosities, and can be mixed with different speed hardeners for various curing times.
There are also different types of polyester resins also, including vinyl ester types. Depending upon whether or not you are doing multiple layers of 'glass, you will need to use a waxed or un-waxed type of resin.
Also, what type of 'glass (cloth or mat)you are using may dictate the type of resin.
For my fiberglass mold work, I used the polyester resin and 1.5 oz. mat. I did also purchase some epoxy, but I have only used for attaching and gluing parts together, not as a fiberglass reinforcement.
Not to send you down the wrong path, I suggest you visit these sites and get the scoop from the manufacturers.
I also found out a lot from this board by doing some searches on "fiberglass" and "fiberglass mold". I believe that Powerrodsmike had some excellent posts regarding fiberglass work.
Hope this helps,
03-04-2010 11:22 AM
mrcleanr6 ok, i worked for a large yacht company for years so i have a pretty extensive background in glass and composites. there is a big difference in polyesters from epoxies. as for cure times, with epoxies there will be different activators so you can change your cure speed to whatever you want. both polyesters, epoxies and pretty much anything with a chemical reaction will generate its own heat causing it to cure faster so ambient temp and activator is a big thing. the faster the reaction the more heat. the thicker the material the more heat it will generate and faster it will kick. too fast and too hot will crack. polyesters are hard and rigid and will spider crack easier but epoxies are stronger by being more flexible without cracking. polyesters for boatbuilding and automotive use is exactly the same stuff although there are many different kinds. everything from fire retardant, winter and summer time, thicker or all depends on what you need. epoxy is harder to wet out the glass since it has no solvent to help it soak into the glass. you really need to work the stuff alot of time and thicker mat is even more difficult so with epoxy you want more thinner layers as it will be easier to work with.
03-04-2010 11:14 AM
BarryK Cboy,
I sent this link to Jim, think his old name her e mrclean, hes an expert on this stuff from the boat industry and knows all the ins and outs.
03-04-2010 10:59 AM
cboy Thanks OLNOLAN and DanielC. Just the kind of info I was looking for. So it sounds like there IS a difference between resin and epoxy.

Can anyone get more specific on set up time (as in minutes/hours/days)? Let's assume I keep the shop 65-70 degrees. I'd like something that sets up within a few hours to over night but could be worked rather leisurely while doing the initial application. Does that sound more like marine epoxy or more like a resin with a slow catalyst?

I just don't like the feeling of being rushed or worrying all the time about the resin kicking, which in that past has caused me to create a lot of waste and sometimes messes up some nicely smoothed pieces as I try to work out any bubbles or voids and suddenly I have little globs of kicked resin sticking all over in that nice surface.
03-04-2010 10:15 AM
DanielC Like OLNOLAN said with polyester resine, you can also mix fiberglass strands, ground fiberglass into the epoxy resin to make a putty.
03-04-2010 10:06 AM
DanielC I have done a fair amount of repair on fiberglass boats. Floor replacement on three different boats.
I use the marine epoxy from Tap Plastics (

Epoxys use only one ratio, and you do not change it depending on the temperature. The rate epoxy cures is totally dependent on the ambient temperature. You can get epoxys that have a different rate of cure, but that changes other characteristics of the set epoxy.
I have been using their "marine" epoxy resin, 314 resin, 102 hardener, it mixes in a 4 to 1 ratio. It will set in temperatures down to 40 degrees Fahrenheit, but at those temperatures it will take overnight to cure.

Epoxy resins are more expensive. Epoxy sticks better to old materials. Epoxy resin usually does not have any solvent in it, so it does not smell, and it does not shrink. The epoxy does surface cure much better than polyester resin, although sometimes it can get a very thin oily layer on the surface called an amine blush, that can be removed with soap and water.

Epoxy resins will generate their own heat, and if you mix a large batch, and leave it in a compact container, it will start to heat up, and this will make the cure go even faster, creating more heat, and speeding the cure up even more. You can avoid this by mixing small batches, or immediately after mixing, transfer the mixture to a large shallow pan, like a bread tray, for example.

Obviously, wear eye protection. Latex gloves are fine with epoxy, no solvent in the epoxy to attack the gloves. Acetone works fine for cleaning tools, but acetone will eat through the latex gloves very quickly.

Most food containers work for mixing the epoxy, but you need to clean them out first. You can also use the "painters cups" from the auto paint store with the measuring marks on the side. Polyethylene plastic bags will not stick to cured epoxy. I have stuck pieces of 3/4 thick foam rubber inside a plastic bag to hold fiberglass, and epoxy on overhead surfaces.
03-04-2010 10:02 AM
Fiberglass Work

Hey CBoy, I remember when I got my PHD(read two books at the library)in fiberglass work. While I'm not familiar with using an epoxy with cloth or mat, I thought I would point out a couple of things. Polyester resin for automotive use is different from the marine type. The auto type has an accelerator in it to speed the process due to time constraints in body shops etc. The marine grade resins set up real slow(supposed to be alot stronger). I did a bunch of glass work on my boat years back. I had read about making an epoxy from resin, the English author called it "Potch". A combo of resin, ground fiberglass, fiberglass strands, talcum powder and something else, I forget. I bought all the stuff to make a big batch from an old fiberglass man locally(guy was a talking encyclopedia on glass work). Used it to "glue" a new transom insert into the boat. Didn't think it would ever set up(took a month). Its been in there for nearly twenty years now. If the super fast setup time is your main aggravation, you might try using marine grade resin. I found through experimenting with varying amounts of MEK hardener using marine resin, you can nearly predict the setup time. I also found that mixing smaller batches in plastic butter tubs reduced waste. Washing your rubber gloves immediately after a batch in acetone allows them to be reused a bunch of times. Food for thought. olnolan
03-04-2010 08:50 AM
Resin vs. Epoxy for fiberglass work?

I've been reading some boat building web sites and they talk a lot about using epoxy with their fiberglass cloth rather than resin. Is there a difference or is this just a matter of wording used by the boat guys vs. the car guys?

I'm particularly interested in knowing what difference (if any) there might be between the pot life of epoxy vs resin. I've always hated working with resin and glass because it sets up so quickly, creates a terrible sticky mess, and is quite unforgiving if you are not very very speedy.

Any boat builders which knowledge about potential differences?

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