Applying Epoxy and Fiberglass - Part B

In Part A we reviewed the procedure for wetting out cloth on the outer hull of a cedar strip boat. This could be a canoe, kayak, or rowing boat. To review the first part, go to Part A. We left off with the cloth coat wetted out and cured.

Sanding the Cloth:

In order to have the epoxy grip and adhere, we need to sand the previous coat. If you have gone beyond 14 hours at 70 degrees, then you should sand. It actually helps to hand sand the cloth coat before proceeding. This knocks the epoxy off the high parts of the weave of the cloth and makes it easier to do a good job applying the epoxy. Note that I said HAND SAND. You should sand with 100 grit to scuff up the gloss. Vacuum and go over the hull with Scotch Brite. This will put little scratches in the gloss of the low parts of the weave. Vacuum again and you are ready to proceed. Do NOT use any solvent during the epoxy process as it can contaminate the surface.

Keel installation:

If you are building a rowboat or canoe and installing a keel, then now is the time to epoxy it to the hull, while the hull is still on the forms and will not be distorted by the keel. In the case of a canoe, the keel can tie into the outer stems at each end or the stems can fade into the hull and the keel can be high in the center and taper toward each end. Either way, you will lay the keel in a bed of thickened epoxy and attach it to the hull by strapping around the strongback.

Bias cloth:

If you are going to apply bias cloth or an additional layer of cloth on the outer hull, now is the time to do it. Cloth cut on a 'bias' means strips of cloth that are cut at 45 degrees to the length of the cloth. Cloth cut like this will wrap around fairly sharp corners without protest, bunches, or bubbles. Refer to our DVD, "Applying Epoxy and Fiberglass" to see a graphic version of this.

First Filler Coat:

The so-caller "filler" coats of epoxy are needed to fill the weave of the cloth such that when you sand, you won't be sanding into the cloth itself.

Since you are applying epoxy to the hull on the bias or additional layer of cloth on the bottom, you could just continue and apply a filler coat to the rest of the hull. Come back 6-8 hours later and you can scrape the edges of the secondary layers of cloth easily because it will still be green.

NOTE: If you are building a kayak, I would not apply additional filler coats to the hull at this point. The reasoning for this is that you will be applying additional coats of epoxy to the hull later when you attach deck to hull and apply bias cloth along the sheer line. So, we think it makes more sense to wait until the deck is glued to the hull and bias is applied to the sheer before applying additional filler coats.

I find it easiest to apply filler coats by applying with a brush, rolling with bristle roller, and then tipping that area out with the brush to remove as much of the roller stipple as possible. Note that you will always have some bumpiness that will needed to be sanded out.

Subsequent Filler Coats:

Let the first filler coat cure for 6-8 hours at 70 degrees, scrape the drips, drools, and imperfections and you can apply another filler coat right over the first while it is 'green'. If you can make a mark in the last coat with your thumbnail, then it is still 'green'. If not, you should hand sand with 100 grit to scuff up the gloss with scratches that will grip the next coat and create a mechanical bond. I always scuff with 100 grit because I am concerned about the epoxy bonding in the green stage.

It is difficult to say how many filler coats you will need. In the vertical areas, the epoxy may sheer down and it will be difficult to build up coats. You may need to add more in just those areas. Yes, you can do this so you aren't adding a complete coat of epoxy to the hull and adding more unnecessary weight.

Again, this sequence applys to the outer hull of a canoe or rowboat. If you are doing a kayak, leave the cloth coat until the deck and hull are together and then you will proceed with this process.

Wear protection:

When you sand any of these layers of epoxy, wear eye protection and an organic vapor filtered face mask. The epoxy dust at this stage is potent; you should NOT be breathing it. It is also prudent to wear long sleeves so the dust isn't all over your arms and torso. When sanding, use a sander with some kind of dust pick up. Sometimes you can modify the dust pick up on your sander by attaching a hose directly from your shop vacuum.

Every time you sand, you need to vacuum with your shop vac. Pay attention to the filter in the shop vacuum as it can get clogged with the cedar dust, shavings, and epoxy dust and become ineffective.

Initial Sanding:

So, once you have two or three filler coats on and it is looking fairly covered, let it cure for a couple of days. You can accelerate the cure by increasing the temperature. Either raise the temps of your work area or warm just the hull itself by installing 'clamp' lights under it so that the warmth from low wattage light bulbs rise and spread their warmth throughout.

Using a random orbit sander with and interface pad topped by 60 grit sandpaper, sand the whole outer hull being careful to just hit the high spots. Don't grind into things, don't concentrate on any one spot. You will see glossy spots left where there are low areas. Don't be tempted to grind into them, they will just get lower and you will be increasing the problem. Leave the glossy spots till you are done, take a 100 grit sand pad and scuff each one up.

Smoothing Coat:

Now you are ready for what we will call the smoothing coat of epoxy for lack of a better term. At this point you have the outer hull sanded with 60 grit and the low spots hand sanded/scuffed with 100 grit. This coat will go on much easier, more like varnish. At this point there is a possibility of fisheye. This is a condition where when you apply the new coat of epoxy, it separates and won't stay smooth, kind of like oil and water. The epoxy company tells us that it is because of an electrostatic charge, either to the hull, the plastic epoxy containers, or both. They tell us to add a half teaspoon of colloidal silica to this coat of epoxy to change the surface tension of the epoxy. This does seem to work. The real downside here is that if you don't realize the hull is charged and apply the coat of epoxy, the fisheye problem will result in your sanding this coat all off again.

As a further preventative to the fisheye problem, I would recommend grounding your hull somehow. Take a copper wire and run it from the hull to a ground pipe to disperse the electrostatic charge. Think of this as when you are sliding around on your carpet in your stockinged feet and when you touch a piece of metal you get a mild shock. Same thing. The sander and later the vacuum have generated an electrostatic charge that we need to disperse.

Apply this coat of epoxy with roller and brush but in this case be very careful to brush out the roller stipple immediately after rolling each section. Waiting will make it more difficult as the epoxy will thicken as it starts to cure.

As an additional method to get this coat smoother, have a hair dryer ready (a paint removing heat gun will also work if used judiciously) and as soon as you have one half of the hull done, flash over it with the hair drier on high (paint remover on LOW) about 8" away from the surface. It there are bubbles, this will serve to burst them and smooth the epoxy.

Final Sanding:

After the smoothing coat has cured for 3 days or so, you can sand again. This time start with 100 grit as you don't want to put deep scratches in the epoxy. Further sand with 180 grit, then with 220 grit and you are ready to varnish. See the varnish tips at the back of our Strip Building Notes for some good information on how to do a decent varnish job.

If you want to clean your hull between sandings, you can use water with ammonia. Wash with cloth, dry with a towel and let air dry for at least a couple hours before proceeding. You can clean epoxy with lacquer thinner, but I don't recommend it as the lacquer is volatile and not a good thing to be exposed to. Do NOT use mineral spirits or acetone in the epoxy process. They will cause the cured epoxy to reject the next coat. You will be using mineral spirits to thin the varnish but this is only after the epoxy process is done.


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