Canadian Canoe Routes

Epoxy selection advise
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Author:  Michel vd Hoven [ August 15th, 2006, 5:32 am ]
Post subject:  Epoxy selection advise

I'm at the point of purchasing the epoxy for my woodstrip Osprey.

My local epoxy shop Polyservice has advised me that for wooden boats (canoes included) I should use a low viscosity epoxy base coat first prior to applying the glass with the standard epoxy. Reason for this, so they told me, is that when not applying a base coat with low viscosity epoxy the wood will suck too much epoxy from the wetted glass into the wood.
They claim that when using a base coat first, a better bonding between glass and wood is obtained, less bubbles will be present, completely wetting out the glass is easier (since wood doesn't such epoxy from the glass).
The low viscosity epoxy for the first coat is called "injection epoxy";
The other coats of epoxy will have "standard" viscosity;

Has anyone applied this method before? Any comments?

My major concerns:
* after applying this low viscosity first coat I will have to wait until coat is hardened completely before applying the glass. Otherwise glass will stick to first coat.
* this means I'll have to sand the first epoxy coat before applying the glass;
* applying a new epoxy system to an already cured base coat will most likely not be as strong as when applying epoxy directely to the wood (less bonding).

By the way, the epoxy I intend to use us sold under the name Poly Pox, which is the name used by Polyservice.


Author:  jjoven [ August 15th, 2006, 6:16 am ]
Post subject: 

Some precoat, some don't. I use a low viscosity brand for wet out and filling the fiberglass Last time I didn't precoat and haven't noticed any difference.

If you don't precoat when wetiing out make sure you apply plenty of resin to the glass and let it soak in. Don't "starve" the glass.

Author:  RHaslam [ August 15th, 2006, 9:02 am ]
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I generally don't pre-coat. Just use a brush and lay the epoxy down, squeegy off the extra after it's had time to sink in. I did pre-coat the interior hull of my last canoe, as it is usually the interior where bubbles might occur. The pre-coat took very little epoxy, and a light hand sanding to just get any rough spots or edges is all that is necessary.

I didn't pre-coat my Osprey...but i did lay a long strip of glass along the inside of the knuckle before I applied the interior was probably not necessary, but I just wanted a little reinforcement in that area. So not sure what the definative answer is...some builders swear by a pre-coat, others claim it just adds some extra weight.

Author:  Dan. [ August 15th, 2006, 9:09 am ]
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I pre-coater the inside of my ranger... The layup went much smoother than the outside where I did not precoat. With precoating you do not have to worry about starving the glass, so it is easier not to over apply resin, thus saving both weight and money.

This was my first boat, so I am far from an expert.

Author:  Komatiq [ August 15th, 2006, 10:32 am ]
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Michel vd Hoven Wrote:
The low viscosity epoxy for the first coat is called "injection epoxy";
The other coats of epoxy will have "standard" viscosity;

Hi Michel, I use both methods depending on the viscosity of the epoxy I'm using (tried quite a few). They likely advised a lower viscosity for the initial coat so you would be sure the epoxy saturated the wood and cloth with a first coat. Often using a slower or faster hardener will effect viscosity to some degree but some resins start off lower.

If you want a little more insight into different viscosities there is a really indepth test done on a slection of epoxies on the website this link should take you to. ...(although the resin you mentioned isn't tested.)
Here's another from the same sight discussing woodcore/epoxy construction. and it's benefits.
You may well come across some info that you'll find interesting and helpful to your project.

The lower viscositiy resins are being labelled "injection epoxy" because they are flow far easier and work better for resin injection, vacuum bagging and vacuum infusion. The higher viscosity epoxies either flow too slow for these methods or require too much time to complete the wet out before starting to cure.

In theory, precoating will insure a better bond to the wood because it is able to penetrate the wood surface further before the curing process kicks in.

Hope that helps a bit...... :wink:

Author:  frozentripper [ August 15th, 2006, 10:33 am ]
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I didn't precoat either the outside or the inside, with a low-viscosity epoxy and had good results. Squeegeeing the epoxy off after about 20 minutes should ensure that enough has soaked in and there won't be an excess which could create runs.

Temperature is probably more important than the pre-coat... too low a temp could mean that the epoxy will be thicker and it won't soak in quickly enough before squeegeeing off. Some epoxies advertise that they can be applied at 10C, I don't believe that applies well to building strippers and working at that low a temp is risky. A builder and teacher in Sault Ste. Marie keeps his workshop warm, 30-35C and the fiberglassing in his class always works out... I kept my temps about 23-25C.

I could see the need for a precoat if the epoxy's thick, with thinner low-viscosity, precoating probably just adds on unnecessary work.

Author:  Arne [ August 15th, 2006, 9:12 pm ]
Post subject:  Epoxy pre-coat?

I pre-coated my Wee Lassie, per instructions. I used West Systems 207/105, I think. It worked fine, but I have no other experience to share (over wood).


Author:  Michel vd Hoven [ August 16th, 2006, 6:08 am ]
Post subject: 

Thank's for all your input so far.

OK, I decided to pre-coat with low viscosity epoxy prior to applying the glass.
That's one question solved.

Second question:
When applying a 2nd coat of epoxy within approx 4-6hrs sanding the first coat isn't required and a strong chemical bond will (hopefully) form. But... will the first coat be "dry" enought after lets say 4 hrs. to safely place the glass on top without the glass sticking to the uncured first coat? Does the first coat already feel dry and can the glass still be moved, wrinkles removed etc.?

Hope someone have experienced this on first hand.

One more question: how much epoxy should I need for a solo canoe like the Osprey? Canoecraft indicated 2-2,5 gallon.. which is somewhat like 8-10kg? In my country epoxy is measured in mass units, not volume units.
I guess I should order 3kg of low viscosity "injection" epoxy and total 7,5kg standard epoxy.


Author:  RHaslam [ August 16th, 2006, 7:39 am ]
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Most low viscosity epoxies will not be dry within 4 hours. Don't worry too much about the chemical bond thing. If you seal the hull, let it dry and give it a light sanding. Don't try to put the cloth on if the expoxy is still tacky, it will not be fun. You will have to give the sealing coat a light sanding in any case, because it will probably pick up some edges......By light sanding, I mean 120 0r 150 just held in your hand and quickly run over the entire hull. Then wipe the hull down....I use tack clothes, some people say don't use them cause the oils will effect the epoxy, etc. Nine boats later I'm still using them.

Does that 2 gallons include the seal coats you are doing? Two should be enough if you are only doing the wet out coat on the inside hull. If you are going to do fill coats on the inside, it might be close.

Author:  Arne [ August 16th, 2006, 8:05 pm ]
Post subject:  Epoxy again

For a Kevlar layup, I precoated the form (plug), layed up the Kevlar (it's a little harder to soak through), then put a layer of 'glass, and a "football" over of 'glass over that. This is all done in one step with a slow curing, low viscosity epoxy. One of the advantages is that you can move all three layers around fairly easily. I would not do this over wood (one step); as many have said, the 'glass could get starved if you're not careful. See

I used the same slow-curing, low viscosity epoxy for the whole works. I think you get a better fill coat, and a finer finish coat. Put it on with a roller and smooth it out with a foam brush.

The one on the left has just had the fill coat applied. The one on the right has the finish coat applied. Working time is at least 3-4 hours. It will be mostly cured overnight.


Author:  Michel vd Hoven [ September 25th, 2006, 5:57 am ]
Post subject: 

... just an update from my "Osprey building progress"...

Last weekens I epoxied and glassed the outside hull of my new Osprey - temperature 22-24 degrees Celsius during day and night.

Used low viscosity epoxy (Poly-pox injecteer) as a first layer, waited for 7 hours for the epoxy to feel "dry" enough such that glass doesn't stick to the epoxy. Layed out the 160gr/m2 (4,7 oz/yd2) glass on the uncured epoxy and wetted out the fabric with standard viscosity epoxy (poly-pox THV500+355 hardner). After 4 hours applied the fill coat epoxy and additional 5" wide glass layer over the stem area. After another 4 hours applied a final coat + a second 4" wide glass layer at the stems.

We did not sand between the layers, since epoxy was green enough to obtain a chemical bond. Everything turned out to be just fine, at least it seems like that...
At 1 location on the side of the hull we had some air bubbles at the tumble-home area... I think this had to do with the small electrical heater which I placed on the floor close by this area. Unfortunately we noticed the bubbles at a stage at which epoxy already was starting to cure, and couln't be squezed out anymore. We had to cut the glass locally and added a "repair patch of glass".

we consumed approx. 400gram of low viscosity epoxy, and approx. 2,2kg of epoxy. In total little more than 3kg of epoxy/glass.

Now we will let the hull cure for at least a week before we will sand the outside of the hull. After sanding I intend to paint the outside of the hull with a 2 component polyurethane DD-coating (after finishing the inside of the hull) since I have used a couple of poor looking flat grain strips which I don't realy like.

Hopefully I'll be able to post some pictures soon.

Michel, The Netherlands

Author:  jjoven [ September 25th, 2006, 6:04 am ]
Post subject: 

all sounds good. One comment. Before sanding I would wash the hull with a weak ammonia solution to remove any blush. Then sand and wash again before applying the polyurethane finish.

One week wait fro epoxy to cure might be OK. Two weeks would be better.

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