Applying Epoxy and Fiberglass - Part A

When we built our first cedar strip boats, we realized that the epoxy/fiberglass application was the most challenging part of the boat building experience. Many first time builders have some woodworking experience and a certain comfort level with that part. So we decided to produce a video showing the epoxy application because we felt it was much easier to comprehend the process by watching experienced builders do it rather than read paragraphs of technical instruction.

That video, now in DVD format, has been very successful in providing good information to thousands of cedar strip builders. However, there are some points that I would like to make about the chemical application process. If possible, watch our video “Applying Epoxy and Fiberglass” before proceeding.

The following description of this process is the way we currently recommend to fiberglass cedar strip boats. Although the general descriptions given in the DVD are still very current, we have come up with new techniques and descriptions of the procedures that will help you do a better job. Read on....

Here are many of the factors you need to be aware of and deal with:

Workshop temperature:

Your workshop should be at least 60 degrees for 36 hours or more before you plan to apply epoxy. A temperature of 75 degrees is ideal. If you are working in a garage, basement or unheated space you may be able to drape plastic from the ceiling to make a smaller ‘room’ to minimize how much you will need to heat. Additionally, you can use clamp lights fitted with small bulbs under the hull; the bulbs warm the air and it drifts upward to warm the hull. In this way you can warm your work instead of the whole area.

Epoxy temperature:

You should warm the resin and hardener for about 20 minutes before mixing. (OK, you don't need to do this in the summer if your workshop is at 80 degrees.) You can do this by putting the jugs in a hot water bath in the kitchen sink or what I do is to have an inexpensive ceramic heater (purchased at a big box store) behind the jugs on my workbench. Warming the separate components first does several things: first, the chemicals flow through the pumps better. Second, the two components mix together better. Third, the warmth lowers the viscosity so the mixed material wets the cloth out faster and easier.

One might think that warming the resin and hardener is flirting with disaster because of the inherent exothermic reaction issue that these chemicals have, but if you mix small batches you won’t have a problem (see next topic).

Batch size:

I recommend working with small batches. This keeps the epoxy fresh and thin. The longer the mixed epoxy is in the ‘pot’, the thicker it becomes. MAS Epoxy is dispensed with pumps that produce 8 ml and 4 ml for a total of 12 ml. I would normally mix no more that 10 or 12 pumps of each, about 4 to 5 ounces. The System Three pumps produce an ounce and a half for each swipe. I would not mix any more than 4 or 5 pumps of each for a total of 6 oz.

Mixing time:

You need to mix the resin and hardener together thoroughly before use. I usually mix for about 2-3 minutes. Failure to mix thoroughly will result in sections of your hull never curing.

Hull preparation:

Before mixing any epoxy, the hull needs to be faired (sanded) up to 80-grit with a random orbit sander. It doesn’t make any sense to sand finer than 80-grit as epoxy needs some ‘tooth’ to grip into. If you sanded to 220 it could be too smooth. Once your hull seems to be ready, vacuum thoroughly and wet the hull with water. Let dry overnight and you will find that the grain has risen. If you see any obvious scratches going against the grain, now is the time to deal with them. Hand sand the raised grain keeping your strokes parallel with the grain using 80-grit. Vacuum and you are ready for the coatings.

Filling gaps:

As far as I know there is no wood filler that will work well in this situation. You may find wood filler that looks like the cedar and once sanded, makes the gaps disappear. However, when you apply epoxy the wood around the gap will darken and the filled spots will be light colored in contrast. For our fool proof method, read on….

Coating procedure:

Think of the epoxy application as three separate entities:

1. Sealer coat. This is a thin coat of epoxy to seal the wood pores and start to fill minor cracks and crevices. You will brush or roll a thin coat of epoxy onto the raw cedar and then immediately squeegee off the excess. Here is where you deal with the gaps: as soon as the sealer coat is applied mix some sanding dust into the epoxy and use an applicator to mash this into your gaps. If you have gaps in white cedar you can use just a little wood flour (sanding dust). Gaps in red cedar need a mix with more wood flour; the more wood flour the darker the mix will get.

Let the epoxy cure for at least 24 hours at 70 degrees. You will see some glossy areas and other areas that are flat and dull looking. This is normal. Hand sand to scuff up the surface a bit and then vacuum.

2. Cloth coat. Next you will wet out the cloth with mixed epoxy according to the description below. You want only enough epoxy on to ‘glue’ the cloth to the wood hull, no excess.

3. Filler coats. After the cloth coat has cured for 24 hours or more, hand sand again with 100-grit to scuff up the gloss, vacuum and you are ready for the filler coats. Mix small batches of epoxy and this time you will brush it onto the hull and then roll it to create a consistent film. Ideally you want to get as much epoxy on the cloth as you can without drips and drools. The reality is that the steep areas will tend to sag, drip or drool so you will be forced to minimize the amounts in those areas. The purpose of the filler coats is to fill the weave of the cloth so that when you sand you will not be grinding into the cloth.

While the filler coat is ‘green’, that is within 24 hours, scrape any drips or drools. Let each coat cure for 3 days and sand before applying another coat. You will generally need at least two filler coats and sometimes three or four in some vertical areas.

Batch application on cloth:

When wetting out the ‘cloth coat’, each batch should be processed thoroughly BEFORE mixing another batch. There are three steps to this:

1. Apply the epoxy mix onto the cloth either by brushing or by pouring and then spreading with a yellow applicator.

2. As soon as the cloth appears to be wet rather than white, use the yellow applicator to squeegee excess epoxy onto virgin areas of white cloth. You want to get as much epoxy out of the cloth without making the cloth ‘white’ or starved for epoxy. The cloth should lay as flat as possible on the hull. You will find that there is a technique for how much pressure to apply to the applicator and what angle to use. The ‘sweet spot’ will come with some experience.

3. Use the epoxy roller to roll the section you just did. This will blend any trailing edges left from the applicator.

Only mix another batch after the steps above are complete.

Start in the center of the hull and work down to the sheerline and out towards the ends of the boat, alternating each batch. You may need to cut the cloth where it overlaps the outer stem at the bottom of the boat. Different designs have different shapes so you need to trim the cloth so it will lay flat in this area. Where the cloth goes past the stems or hangs beyond the sheer line, trim it to within an inch or so. Excess cloth overhanging the edges can tend to pull away from the hull.

Let the cloth coat cure overnight. Use a razor knife or box cutter to trim the excess cloth off, up to the sheer line and right to the stems. Use the fine side of a wood rasp to knock down the edges of the cloth at the stems. It is much easier to do these trimming and blending operations while the epoxy is in the 'green cure' stage. Once it cures for 3 days or more it becomes very hard and brittle so that cutting and rasping becomes more of a challenge.

More to come, filler coats, sanding, etc; go to Part B.


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