Polyester vs Epoxy

hi everyone,

Before starting a thread dedicated to Open 60 at 1:20, I have a (some) question(s) that need an answer : why do i, across all the R/C forums i’ve seen, only find people using epoxy resin to make their hull ? Isn’t it faster and stronger to use polyester one ? Isn’t polyester resin used in real boats & yachts construction (1:1 scale) ?
So i asked Claudio few weeks ago.
He told me that it could be used but it stinks and shrinks.

So i decided to try. In this test i used the same weights/m² of glass fiber that i would on a hull : 1 layer 160g/m², 1 layer 80g/m² and 2 layers 44g/m². Each sample weigh 7g (4 layers about 18*20cm. Didn’t check but the 8 layers have been superposed and cut at the same time).

one sample will be stratified with epoxy, the other with polyester.I found a polyester resin that can be used in stratification with glass fiber and polymerise in 50mn. So i hope it won’t shrink… We’ll see.

First answers: Claudio was wright (again): polyester resin smells real hard !
You have to double the weigh of resin (polyester) Ex: 7g glass fiber+ 14g resin ( could be a handicap…)

lets check it on pictures:

Step two :

Stratification. I added some packaging tape on the bottles to be in the same conditions it would in hull making.(wasn’t necessary on plastic bottles but…)
Add 14g polyester resin and 7g epoxy.
Waiting 1 hour to be sure besause i had to open the windows and the temprature today is only 10°C (eastern France)



The polyester print is now removed (quiet easely). It looks more rigid than epoxy (have to wait 24 hours to compare exacly with the same shape… but it realy looks much more rigid…) and it should weigh 21g (7g GF/14g resin) and it results to 18g only. Not bad for instant.
Will be out for the week-end, so we’ll discover the next step and final comparison on monday.


Remember that “stiff” does not always equal “strong”. There is a relationship between the fiberglass reinforcing and the resin, fiberglass will elongated approximately 5% before rupture with 100% elastic return up to rupture. A good polyester resin will elongate just over 2% before failure at 9400 pounds per square inch, a vinyl ester resin will elongate over 4% and provide a tensile strength of 11,800 pounds, and most laminating epoxies had elongation’s in the 5 to 6% range and a tensile strength near 12,500 pounds per square inch.

Using a resin that will “stretch” as much as the fiberglass that is reinforcing it, will give you a much “stronger” layup. This will let the fiberglass carry much more load, and that’s it’s job!

Epoxy resin may not smell as bad as polyester, but it still has bad long-term health risks. Try not to get it on your skin, and if you do, do not wash it off with acetone! Acetone will penetrate your skin and bring the resin along with it. If you ever become sensitized to it, you can never use it again! Wear gloves when laminating and a respirator when sanding.

If you are interested in “technical” reasons to use epoxy, might I suggest you email Gougeon Brothers (WEST System) and ask them for their technical bulletins and also on their laboratory tests of differences between epoxy and polyester. Also keep in mind there are epoxies for virtually all DIFFERENT uses, and picking the right one - or the wrong one can provide a great difference in their performances. Since Gougeons developed their epoxy formulas, and are one of very few “Builders” as well as manufacturers, I put a lot of importance in their products, their testings and their experiences. They were one of the leading epoxy builders that showed why epoxy was some much more superior to polyester resins. You can buy cheap epoxy that will never harden - and stay soft. Since they have many boat builds by themselves, plus provide references from many home and other professional builders, they have major experience.

I primarily use their products because they don’t smell, and I grew up using their products and appreciating the results. From sail board, to “real” boats, to our model boats, I know what to expect from the product. I have yet to feel comfortable mixing up a batch of polyester resin and hoping it won’t cure too early. Their concept was to saturate the wood pores with epoxy and only use glass, carbon or Kevelar for strength - allowing the epoxy to seal the wood from air, mositure and rot. (WEST System = Wood Epoxy Saturation Technique) I have been lucky enough to have lived clsoe to them, seen their creations, and reviewed many of their tests - and even without the friendships - I will continue to use the products.

Again - visit their web site, read/request technical bulletins, and then make your own choice.

Regards, Dick

Hi Paulin,

There isn’t going to be much of a weight difference between the two samples as they are both a simple hand layup. The benefits of using epoxies begin so really shine when you move up the laminating technology scale and start vacuum bagging etc.
Compressing the fibre, increasing the fibre/resin ratio helps to get the best from the materials and reduces your boats final weight.
Having built a Open 60 style yacht before, I have found that weight is your biggest issue. Having a really wide yacht with big ‘flat’ sections means a much bigger amount of cloth + resin goes into making the hull & deck, plus you need more structure inside the yacht to stop the hull deflections particularly in the stern area where its flattest…
For example - My ‘Open 60’ 1 metre 100% carbon pre-preg hull, autoclaved, still weighed in at almost 300 grams. 1 x RC 200g and 1 x 100g unidirectional.
My 1.2 mtr IACC wasn’t anywhere near that weight and it was a epoxy wet - layup vacuum bagged effort using a 300 GSM Bi axial carbon cloth.

I would always go with the epoxy as the best route, and look into finding a cheap vac pump on eBay… Mine was €80. You wont regret it. Just don’t forget to post-cure…!.


Isn’t polyester resin used in real boats & yachts construction (1:1 scale) ?

Just thought I’d add this too:
Yes, Polyester is used on 1:1 sized yachts for production and cost effectiveness… but NEVER on a pure race yacht where its all about the strength/weight ratio and winning is all that counts and not the budget!

Hi guys,

Thanks for all those precious informations. I haven’t understand everything yet ( because of my english level ) but will translate the technical therms thanks to Harrap’s shorter…
Maybe i’m using the wrong Epoxy resin. The one i’m using ( Vosschemie ) was found in a “Do-it-yourself centre”. I will try a print with WEST System products and i could see the difference.
I’ll tell you.

I’m not affraid concerning the strenght with Polyester resin because i think 9400 pounds per square inch and an elogation of 2% max should be quiet sufficient for a 91.4 cm lenght and 29 cm wide hull. ( i may be wrong )

Concerning the weight of both samples, it is wright that there is no big differences : 18-19g for polyester, 17g for epoxy…

If strenght is enough and weight is quiet similar (with this lamination technic) i would opt for stiffness… That also means less reinforcements of the hull and so a gain of weight… (maybe)

I’ll go watching the technical articles of WEST System and will try a third sample with their Epoxy resin.

I’ll come to the vacuum bagging technic one day but i’m just mastering the hand layup technic, so i’ll go step by step.
About the post-cure : does it means warming the hull at 120°c during 4 hours or applying a final coat of resin when it’s dry ?

Thank you all,

Hi, Paulin,
what you call stiffness ist not a matter of resin, but a matter of fibres. The glass, carbon or kevlar makes the strength of the laminate. The resin “glues” the fibres together. Roughly speaking, resin gives no strength, only weight. Cured resin without fibre reinforcement is quite brittle - polyester more than epoxy. This is why you need the fibres. Weight ratio resin/fibre should be 1:1 for a good performance, a typical hand made laminate will have somewhat between 1/2 to 1/3 fibre content. It depends on your skills.
For our models the strength of glass fibres is more than enough. Two layers of 105g/sqm glass filament fabric are sufficient to make a RG65 hull. Of course, such a hull feels floppy without deck, but the only purpose of the hull ist to keep the water outside. An IOM will require something like two layers of 163g/sqm fabric.

Using Carbon (not allowed for IOMs) you will get more stiffness (the hull feels less floppy) and a better look, but the thin carbon fabrics are extremely expensive (1 sqm 93g fabric is about 95€!). Due to the better stiffness of the carbon fibres you will get more stiffness at the same weight or lighter hulls with same stiffness compared to glass. The most extreme hull I made so far for a RG65 was from one layer 50g/sqm glass fabric and one layer 93g/sqm carbon fabric. The weight of the empty hull (without deck) was less than 40g.

Concerning the resin - yes, polyester is sufficient, but it smells strongly and cures very fast. Epoxy resin ist more convenient to handle. However, you have to have a look on the resin and hardener data sheets. There are only a few resin/hardener combination which will cure at room temperature and a lot that will not (or not completely) the latter ones need curing at higher temperatures. The temperature needed is depending on the resin/hardener combination. For the usual ones, 50 to 60 °C is a typical value. Slow heating, holding the temperature for several hours and a slow cool down is necessary to get the optimal perfomance.

However, using e.g. the R&G resin type L with the hardeners L or S, will allow to make good hulls without tempering, but tempering will result in a better perfomance. R&G is a German distributor. This type of epoxy is the one I use for several years now, but there are other , use can use, too.

Hi Paulin,
I confirm all what said by Haegar

hi Joachim, Claudio and others,

Thanks for this clear explaination ! The first and ever RC boat i’ve built and sailed is a 75 cm long, balsa planked structure with 2 glassfiber coats. So it is very stiff, strong… and heavy (1500 g overall) but it works.
The only GF hulls i’ve seen were pictures and didn’t look floppy. So when i removed the first print of my IACC 120 few months ago, i’ve been surprised to see it getting out of shape. I fought something was wrong during the building : bad resin type, wrong mix, wrong temprature… This hull was made of 4 layers of 80 g/m² and weighed about 240 g. I added 1 layer of 44 g/m² and 1 layer of 25 g/m². It is now very stiff but weigh 350 g (resin + fiber + filler+ paint.)
I didn’t knew that once the deck was on it rigidified the whole structure, so i will try a new print with West system or R&G resin to keep a moderate overall weight.
Concerning the Open 60, i’ll just put some reinforcements inside the hull because it’s very wide and flat : 3 or 4 bands of cloth (2-3 cm large) widthwise from the middle to the back and 2 or 3 lenghtwise. Should use 160 g/m² GF.


:graduate: Hi Paulin -

often model boats are overbuilt. A friend of mine in the U.K. asked me one time - “are you going to ride in it or step on it? If not, build it light and just to keep water out.” Model boats, as you have now found out, gain much desired strength when the deck completes the assembly. You still will need to build a hull that doesn’t distort in shape, but at the same time decide how many layers of cloth you really need. That will only come from experience - building lighter each time until you reach failure. Then back up one step. :lol: :lol:

Hi Dick,

Thanks for the lesson ! I wasn’t looking from this point of view, but you’re right. I will wright your motto : “building lighter each time until you reach failure. Then back up one step.” on my workroom wall.

I have a question for Claudio : I have calculated the displacement of my open 60, that is 1975 cm3 (not bad…) and i’m searching for the CC with his method. I not sure of the wright way of calculation and found two results : 56.96 cm or 51.66 cm from the first section. Could you take a look and tell me ?


Sorry, didn’t work… try something else.

Remark : this question is out of topic, nothing to do with epoxy & polyesters !

nevertheless, the distance betwen frames is not given, but I found in the calculations the number 5.3.
According to my understanding and what it is foreseen in the ‘trapezoidal rule’ is that frame ‘0’ and frame ‘10’ have moment multiplier = 0.00
I suggest to review the computations eventually using only 11 frames from ‘0’ to ‘10’.
The simplest way is to cut the ‘Curbe of areas’ in a simple bristol and hanging that to a needle in order to trace the crossing lines.
See attachment copied from one of my books

Hi All
Does anyone have a conversion chart for standard vs metric for cloth, ie. grams/sq.meter vs ounces/sq.yard? Actually, there should be a number I could multiply or divide by to convert.
Second, Dick, ole buddy, since you have an in, do you think you could find out the recommended post cure treatment from West Systems? If there is one?

AllI had to do was Google. The magic number is 33.906(call it 34). Multiply oz/sq yd by 34 to get g/sqm, divide by 34 to go the other way.

I think I found the answer to the post cure question too.

About halfway down the page. It says a minimum of 120 degrees, that’s easy to reach with a cardboard box and a lightbulb.

Don, also check Dick’s post here:


Where is is stated that room cure epoxy systems do not require and do not benefit from post cure. Provided by John M Thomas, Technical Advisor, WEST SYSTEM Epoxy

Thanks Hew
I guess this is where I should feel a little foolish since it was me that brought up the whole “Post Cure” thing in the first place. I kind of thought the whole thing sounded familiar. I’ll blame the 1970s