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The Epoxy Test Results

Epoxy Panel exposure after: The pictures have been optimized for faster download but in order to preserve the resolution, the files are still quite large.
  •  Six months (March1 - September 10 2000)

  • Twelve months (March1 2000 - March 2001) - this was the objective of the test, however, things are only beginning to get interesting, so I will keep the panel exposed and report on the development every half year or so to see how much environmental abuse this epoxy stuff can really take.

Let me point out that the comments in this entire page reflect only my opinion based on my experience with the tested epoxies. I am in no way affiliated with any of the manufacturers. I try to be as fair to the different brands as possible because like most boatbuilders I take pride in my work and finding the best material for my kayaks is the whole point of this test. Whether free, cheap or expensive, if it doesn't cut the mustard, I won't use it!

    Viscosity & Fiberglass Saturation
   Speed of Cure
   Blushing - what is 'Blush'?
   Preliminary Review (of Epoxies)
   Thicker or Thinner Epoxy - which is better?
   Miscellaneous -mixing epoxy brands together
   Epoxy pump problems and alternative dispensing system
   Epoxy cleanup


 Viscosity and Fiberglass Saturation 

  Viscosity is a measure of how free flowing a liquid is. The more syrupy and thick the liquid, the higher the viscosity.
This is one of the more important issues in wood strip construction because low viscosity epoxy allows thorough penetration of the fiberglass thus making it completely transparent over the wooden surface. Even though 100% saturated fiberglass is important, a far more important structural concern is how deep the epoxy soaks into the wood. Thick and fast setting epoxy will not saturate the top layer as deep, making the surface softer, less abrasion resistant and more prone to delamination on impact (the break of bond between fiberglass and wood core).

Viscosity in thicker epoxies can be improved by preheating both the resin and hardener prior to mixing. Warming the cans in hot water, on a radiator, or by infrared heat lamps are good ways to lower viscosity. This may shorten the pot life (working liquid stage) somewhat but it will be far easier to saturate the cloth, so the job will go faster. The other side of the coin is that thicker epoxy will make a better filler of the fiberglass weave while the thin resins may take more coats to accomplish the same thing. Check out the fiberglassing section in the kayak shop

  The 4oz. cloth on the test panel is so tightly woven (40 x 40 per inch) that it tested the saturation power of the epoxies to the limit. What I mean by this is that sometimes the fibers running in the warp or weft direction saturate less - let's say 98% - and are more visible. Even though a tight weave fabric is lighter weight (4oz.), it may be harder to wet-out every last strand of it, than loose 10oz. fabric. I personally use the 4oz. 22 x 24 thread count cloth on the deck and this less expensive, tight weave on the inside of the kayak.

Raka Super
MAS Super
West S 207  
East Sys  
West S 206  
System Three warmed for wet-out



I chose all epoxies mainly for clarity so all of them are clear in thin coats as well as under varnish, but they all differ slightly by hue. Thick coats are a different story so check out the full size samples. The only non-colored epoxy is WS 206. The rest ranges from yellows, green, and pink.


 Speed of cure 

If you are into production or want to get things done reasonably fast, you will be happier with faster curing epoxies. If, on the other hand, you feel apprehensive about epoxy setting up too fast or there is no rush to blast through fiberglassing, then the slower types are for you. Some manufacturers give gel time at higher shop temperatures (up to 77°F) . My shop is 65°F.

Some epoxy formulations allow mixing of slow and fast hardeners to adjust the setting speed but you will have to invest in two hardeners and spend more time (and epoxy) for experimentation. The tested epoxies that allow hardener mixing are:
  • West System 206 & 205 fast
  • MAS slow & fast
  • System Three slow, medium and fast
  • Raka 350 & 610 fast
With the rest you are stuck on one speed. None of the epoxies were mixed so the test is valid only for one particular hardener. The epoxies below are listed from the slowest to the fastest.

Click on pictures for detail
'Gelling' of System Three (medium hardener). This batch was intentionally thickened for 'pre-preg' construction of carbon fiber rudder brackets. The 'Alien Slime' consistency shows the phase change as liquid starts turning into solid. This happens within a few minutes at this point. One would expect uniform thickening but in reality, denser clumps start to form where the reaction heating in the cup was the highest.

The temperature of the epoxy is around 120°F (49°C) and it starts 'steaming' visibly.

Raka 350
This epoxy is slooooow. It will gel in a few hours but 24 hr after application (65°F / 22°C) it will still have the consistency of hard rubber or toffee, so sanding is quite limited. It takes another few days to achieve hardness so that you cannot make a fingernail mark. I suppose, in warmer climate or warmer workshop it would cure faster. This sluggishness is however balanced by good saturating power into the fiberglass and wood. Unrushed work pace.
This epoxy is quite slow but much faster than Raka and at higher temperatures it is sandable (wet-dry) the next day. Good for first time users because it allows comfortable pace of work. I use it personally, and it is especially nice for inside deck and hull so you can spend more time on removing excess epoxy.
System Three
With this epoxy you will be able to squeeze in two to three filler coats in a day. 24 hours later it is hard and sandable. This epoxy was warmed up prior to glassing to lower viscosity. Here again the speed of cure can be adjusted with the fast or slow hardener. There is faster and slower hardener available. Good efficient speed.
WS 207
Very similar handling and setting properties to System 3 although a bit faster. In four hours the epoxy is gelled enough that a filler coat can be applied. You can do up to three filler coats in one day. In 5hr. a new layer of cloth can be put on without sticking too much. Sandable in 24hr. Ideal speed of cure for me.
WS 206
It turns viscous faster than 207 but the gelling is about the same. In a hot shop you may feel rushed but it is manageable.
East Sys
It sets up like WS 207 but it will be strong and hard as a rock the next day. Hardest of all epoxies tested. Efficient working time.



All epoxies were ordered on-line and only WS 206 and 207 could be obtained locally. The prices reflect the best deals I could find in the winter '99-'00. Shipping cost are not included. To make a fair comparison, the prices were calculated for 1.5 gallons of usable epoxy mixture even though some epoxies do not sell that way or the mix ratio is different than 2:1.

Prices per
1.5gal ( 3.785 L)
East System $68,- Noah's
Raka $78,- Raka
West S 206 $85,- local
System Three $85,- Mertons
MAS $89,- Hamilton
West S 207 $123,- local

The Resources page has links to all merchants.

Some dealers will demand high hazardous shipping fees even though the epoxies are classified as nonflammable and can be ordered from other places without any fees, so 'caveat emptor'. None of the above dealers charge hazardous fees for these epoxies.



When some epoxies harden, a byproduct of the curing reaction rises to the surface appearing as a greasy, waxy film. On contact with high humidity or water, this coating turns into an opaque white smudge (during or even after cure) which turns into dry chalky powder with time. This 'blush' will not wipe off easily with thinners or solvents. It can only be dry or wet sanded after the epoxy completely cures.

This property has a huge implication on the time and amount of labor you need to expend to recoat or for bonding. Blush free epoxies allow additional filler coats and bonding without prior sanding! This benefit also cannot be overemphasized if you ever experienced allergic reaction to fiberglass and uncured epoxy dust.
The way this works is that if you re-coat before the prior coat is completely cured, the new epoxy will dissolve the top layer of the 'old' coat and fuse 'chemically'. If let cure for longer, the absence of the blush (surface contamination) permits a clean mechanical bond. Despite the clean surface, it is a good idea to wet-sand cured epoxy if subsequent bonding isn't done within 24 hours or so. This assures a good secondary (mechanical) bond.

Epoxies that suffer from blushing are difficult to trust (without a lot of intermittent sanding). Filler coats can be applied without sanding but it must be done as soon as the epoxy starts setting or shortly after. Furthermore, unvarnished surfaces (such as the insides of the cockpit) may turn smudgy white with time which is quite ugly, aesthetically speaking.

Raka Perfect
West S 207  
System Three  
East Sys  
West S 206 Bad

Click on this thumbnail image to see the test panel from an angle that reveals differences in the epoxy surface.

This image shows in detail the unflattering look of epoxy blush.

 What is Blush? - technically speaking

The greasy films that appears on some epoxies are largely the salts of amine carbonate. Depending on the type and formulation of the epoxy, amine compounds on the surface combine to various degrees with CO2 (carbon dioxide) and water in humid air forming hydrates of amine carbonate. This stuff is supposedly water soluble but it will not leave without some abrasive scrubbing.
Why do some epoxies blush and some don't and why wouldn't everyone want to make non-blushing epoxies?
It just happens that the amount of blushing is very related to the shape of the amines . These molecules in turn determine the physical properties of the cured epoxy. Low blush epoxy formulations often contain 'cyclic amines'. The geometry of these molecules as well as their 'monofunctionality' (one reactive side only) improve the surface, slow down the cure rate and reduce the strength of the cured mass.
What does it all mean? It seems that very slow, glossy, low blush epoxies harden to solids with lower moduli of elasticity (softer and more elastic) which allows for deformation and better impact absorption without cracking. Faster setting, blushing epoxies are generally far stiffer, harder as well as more brittle. At the extreme of this spectrum lie high moduli epoxies which are never used for clear coating but are unsurpassed in their strength. These epoxies are used in high tech, high strength composites that are post-cured with heat.


 Preliminary Review  

The results of the 'year long' epoxy test are now in. The info in the table below describes what features I like the most and least in each type of epoxy.

A great feature of this epoxy is its very low viscosity and good wet-out. When the epoxy sets it is free from even the smallest bubbles and the coat is clear, shiny, and glossy as glass. Builders who prefer slower unrushed pace of construction will love it. I personally find it way too slow by itself but faster setting speed is possible by mixing in Raka 610 hardener. I haven't tried the faster curing combination, but I will certainly find out. All its properties and the second lowest price make it a likely candidate for my Kayaks.
Great wetout, no blush, adjustable pot life with fast hardener and middle of the road price make for a top notch epoxy.
System Three
This epoxy was new to me at the beginning of the test but its setting speed and overall handling similarity to WS 207 make it an efficient production epoxy. Although there is no apparent blush, it hardens with a mat nonglossy finish. Preheating of the resin and hardener to lower the viscosity is an inconvenience I can live with. The overall quality as well as an excellent price hold much promise for System Three in my workshop.
WS 207
This epoxy has been around the longest and every boatbuilder knows it. It is rather foolproof. Its setting speed, decent viscosity, clarity, and workability make it a great epoxy. In thicker coats it may impart a yellow-green hue to lighter colored wood but the varnish seems to neutralize that. For its overall quality, it carries a premium price.
WS 206
A middle of the road hardener with good overall properties. The price is right too. The biggest problem with 206 is heavy blushing, so if you don't mind sanding it is OK. The other nice thing is that if you are invested in the West System this hardener works with the same resin as the WS 207.
East Sys
Good hardening and handling properties and the price is unbeatable. Virtually half the cost of West System. It is unfortunate that this epoxy blushes severely no matter what environment it is in. If you can live with that, than give it a shot.


 Thick or thin Epoxy? 

This issue is a compromise you need to consider before investing in any 'Epoxy system'.
Thinner epoxies are superb at saturating the cloth and wood quickly and completely but this is where the excitement stops. When the epoxy cures, you will spend more time on filling the weave of the fabric until the cloth is completely buried. The thin epoxy is not very thixotropic and will tend to sag, drip and run and you will need to coat the hull with 5 to 6 layers (2-3 more than with thick epoxies). In general, thin epoxies such as the MAS or Raka will require a couple additional days before the fiberglass is covered to your satisfaction.
All is not lost however, because you can 'prepromote' the epoxy to increase its viscosity for filling the fiberglass weave. Prepromoting involves mixing a very small portion of the premeasured hardener (5-15%) with the total amount of resin. The mixture will thicken a bit after a while but cannot harden because of the 'wrong' proportion of the reactants. When you want to use the batch, blend in the rest of the hardener. The mix will now have the proper ratio of resin to hardener and will proceed to set, as if nothing happened. The only thing you will notice is the increased viscosity, now perfect for filler coats.
If you don't feel like experimenting with 'prepromotion' just mix the normal batch and let it sit until the mixture warms up noticeably. Now you can proceed to coat the hull. The slow, thin epoxies harden really slowly so you will have plenty of time to coat and squeegee.

The faster setting or more viscous epoxies (WS207, System Three, East System) will exact more patience when you wet out the fiberglass because their 'syrupy' nature needs more time to soak into the substrate.
This inconvenience will be repaid many times over by having to apply fewer filler coat. This will save you a couple days of work. These epoxies will keep you on your toes because the amount of time from saturation to squeegeeing is reduced and rapidly thickening epoxy is more tricky to squeegee because air bubbles can easily make it 'frothy' and opaque.

If you have never used epoxy in this way before (clear coating wood), I recommend to go with the thinner types. The learning process will be that much more pleasant.

Click on pictures for detail
This picture shows the fiberglass weave after squeegeeing the first filler coat (MAS epoxy). This is how it should look but it doesn't last long because it was put on top of barely cured substrate only 3 hours later.

The hardener is a blend of 20% fast and 80% slow
This is the same patch of surface as in the picture above only few minutes later. I am puzzled by this myself but it seems like the uncured epoxy wants to repel the new coat in the same way a new car polish beads up water. Good ol' surface tension at work.
This 'beading' actually became an advantage because the next filler coat really filled the deep valleys between the high beads and it saved me from applying the last coat. It took 3 coats in total to cover the deck.


 Miscellaneous - can different brands mix together  

When I tested the different epoxies, I wondered what would happen if leftover batches from different brands were mixed together. At first I imagined that some violent reaction might take place or that the mix would turn into sludge that never hardened. I was very surprised to find out that all three combinations (out of 15 possible permutations) cured completely as if it was one brand.
How? Why? I don't know. My guess is that either the chemistry is so compatible that the hardeners and epoxies can cross-react with each other, although even mixes of epoxies with different resin/hardener ratios (for example 2:1 & 5:1) solidified completely.
The other, more likely possibility is that once each epoxy batch is mixed completely, the resin and hardener molecules bind and begin polymerizing. Addition of another similar substance such as another fully catalyzed epoxy will not disrupt this reaction and both epoxies will be mechanically (rather than chemically) crosslinked. Maybe it is a combination of both.

I have been informed that hardeners and resins (amine based) from different brands could be chemically cross-reacted and fully cured. The proper ratios of resin/hardener depend on EEW (epoxide equivalent weight) and AHEW (amine hydrogen equivalent weight) but this needs to be calculated for a complete cure to take place.

The implication here is that bonding between different types of epoxies (during the 'green' stage) could be successful even without prior sanding. I haven't tried it myself.
This little experiment was inspired by the fact that polyester resins will not adhere to cured epoxies well, because the epoxy inhibits curing of the polyesters at the surface. On the other hand, bonding fresh epoxies to cured polyesters as well as other epoxies is possible.

Epoxy brands blended together (1:1):
WS207 + East System ; MAS + Raka ; WS207 + System Three


 Epoxy pump problems  

Click picture for detail
Epoxy pumps are a great time and labor saver during fiberglassing but many epoxy merchants will warn about testing the accuracy of the pumps prior to using them. OK, so you test the pump and find out that it is inaccurate. Do you throw it out only to find out that the next one you order is just like the previous one? The problem is that the larger hardener pumps ( for gallons and quarts) have a fundamental design flaw in that they can be rigged for two settings. When you unscrew the top and pull out the plunger you will notice a lengthwise split spacer (tube) that prevents the plunger from retracting all the way to the top. This limits the amount of hardener the pump can take up. Check whether the spacer hugs the plunger stem only. If it is, then it is the way it should be (although there is no assurance that it will stay that way).
Sometimes the split tube is forced over a thin collar just behind the plunger making the spacer effectively shorter. This makes the pump deliver more hardener which is what you definitely DON't want!! Pushing the pump hard may easily change the setting from good to bad without you knowing. The solution: Wrap a wire tightly around the spacer before the plunger or make your own spacer with hard stiff tubing.

I would advise against metallic spacer for it may alter the color or chemistry of the hardener in the long run. I haven't tested the hardener - metal reaction so it is a pure guess on my part. If you try it and it works, I would love to know.

Don't feel like messing around with pumps?
Pumps are good only when you work with one or two epoxy types and only certain size containers. If you work with more epoxies, many different size containers OR you mix both large and tiny batches AND you like to mix slow and fast hardeners to customize the cure time, pumps are just a hindrance.

The pictures below show how to make an inexpensive, accurate and flexible epoxy dispensing system. I use this more often than pumps. Ten to twenty cups are enough to take you through the entire kayak fiberglassing process.
First decide on the size of the epoxy batch you want to work with. It can be as big or small as you like. I also chose 2:1 ratio epoxy for this example but you can adopt it for any ratio system.

  . Place an empty cup (a) into your 'gauge' container and mark off the level of the hardener (H1). Pour water to the mark. Pour the first measured portion into another cup (b) and refill the cup (a) back to the H1 level. Now pour cup (b) back into (a) and mark the level on the 'gauge' container. This is the level for the resin (R2). Now your gauge cup is calibrated to dispense 2:1 portions of epoxy.
  The last thing you need are dedicated resin and hardener cups to be placed into the 'gauge' cup for refills to the proper level. Pour the measured portions into your mixing cup so that you can reuse the dedicated cups over and over. Don't forget to mark the containers 'R' and 'H' so that they don't get mixed up. The gauge cup will work well until it gets gooped up with too much epoxy and the cups will not fit in accurately.


 Epoxy Cleanup  

Most people clean epoxy with thinners such as acetone. Because of its toxicity, expense, and fire potential, I try to limit its usage only for cleanup and soaking of brushes or tools that are used immediately. Since epoxies are fundamentally alkaline, (like lye-sodium hydroxide or baking soda-sodium bicarbonate), they can be neutralized by acids such as vinegar. While acetone thins the epoxy (when it evaporates, a sticky film remains) vinegar neutralizes the epoxy completely and with it its toxicity (new chemical is formed, no epoxy remains). Vinegar is cheap non flammable and removes uncured epoxy completely. Because of its acidity, don't use it to clean the hull or surfaces to be re-coated!

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Stoneham, MA. 02180
Tel:  781-481-9261


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Last page update: 29 October 2013