Covering A Canoe
HOW TO FIBERGLASS A CANOE
Read and understand the safety information for the epoxy system you are using. You might want to build another boat some day. Follow these safety rules:
LAYING UP FIBERGLASS CLOTH IN EPOXY RESIN
The secret to a clear coating is to control the film thickness of each layer of epoxy as it is applied. Building up the epoxy/’glass over the wood may be compared to making a mirror. The prepared surface of the wood is the reflective coating, and the epoxy/’glass is the glass, or lens. The lens must be clean, smooth, and of a consistent thickness. It is impossible to apply the epoxy resin in a haphazard manner and sand it back to a consistent film thickness. When the epoxy builds up thick all at once, air is trapped inside and will appear as a white blemish or streak. If it is in the first coat, there is a good chance it will be in or below the surface of the cloth. Sanding will not remove these blemishes without cutting into the ‘glass fiber, thus weakening the structure.
Even buildup will involve a different technique for each coat – each of which accomplishes a specific function and is controlled by specific limitations.
On the outside of the hull, where durability and visibility are prime considerations, three layers of epoxy resin and one layer of ‘glass fabric are used. The ‘glass fabric is always laid up in the first coat of epoxy; the second fills the weave and levels the surface; and the third buries the fabric, giving enough depth of resin to sand smooth without hitting the cloth.
On the inside, consider omitting the third coat of epoxy. This creates a surface that will be less slippery yet will look good. Also, it eliminates about 90% of the sanding of epoxy on the inside, a step that could be expected to take about one day to complete. The trick here is to be extremely careful squeegeeing the first and second coats. The texture of the weave must be consistent in order to look good. Think about this when you are squeegeeing the outside; it is a good place to practice getting an even texture.
WORKING WITH EPOXY AND FIBERGLASS CLOTH
Assemble all materials and supplies, and set aside a block of time when you will not have to deal with distractions, such as the telephone. This is a one-shot deal and deserves your undivided attention.
Roll the cloth out over the hull, leaving about 6" extra at each end. With a person at each end holding the cloth in the middle, pull the cloth back and forth to center it on the hull. While holding the cloth at the middle with one hand, use the other hand to work down to the edge, tugging against your partner every 4 or 5". This exercise will shape the cloth perfectly around the hull. You should be able to manipulate the entire piece of cloth from the end.
Use this technique if wrinkles develop in the cloth during the lay-up. Trying to brush them out with your hand could put a permanent crease in the cloth.
This is a good part of the project on which to have some help. Break the duties down to mixer and applicator. They are both critical positions – although, if it is your boat, you may want to be the one in direct control of the visual aspects by applying and squeegeeing the resin.
Don’t let anybody distract the mixer! Mixing requires a great responsibility and concentration. There is no graceful solution to leaving one shot of hardener out of a batch that is being applied to the cloth. On the second coat, the resin could be washed off, but when it is in the cloth, more drastic solutions must be considered.
Get into the habit of dispensing the resin first, followed by the hardener. Count to yourself or out loud if it helps; one (resin) one (hardener), two (resin) two (hardener), etc. Keeping to a system will reduce the odds of getting lost. If for some reason you have doubts about the accuracy of the ratio, discard the mix rather than take a chance on spoiling the lay-up.
When choosing the number of shots per batch, there are several things to consider. Once the resin and hardener are combined, a chemical reaction begins that produces heat, and this heat produces more heat. The larger the batch, the faster the heat will be generated. As the heat builds, the epoxy loses viscosity and begins to thicken. This thicker resin will not soak into the surface of the wood or the glass fibers as far as the thinner, fresh resin will. You will see this as a lighter shade on the planking, and the ‘glass fabric could be visible. If you feel the resin beginning to heat up, discard it.
Always use fresh resin. Tell the mixer just before you run out so that there will be a fresh batch of resin ready when needed. Applying the epoxy resin to the hull surface will require a different brush technique for each coat.
Suggested Batch Sizes:
FIRST COAT OVER DRY CLOTH
The function of the brush on this coat is to transfer the resin from the can and to spread it over the dry cloth. There should be no attempt to work the resin into the cloth, as this will introduce air bubbles into the resin that may or may not come out when it is squeegeed. Load the brush and spread with controlled but bold strokes. As you work aggressively ahead, keep an eye on the area that has just been covered. Watch for places where the cloth looks dry or starved, and add fresh resin; don’t try to move a patch of resin of questionable age over to it. And don’t mess with the runs too much; they will be removed later with the squeegee.
This is one place where working aggressively with disregard for neatness is the most appropriate thing to do. The fresher you can keep the resin at the edge, the more consistent the colour will be.
When you dip the brush, there will be resin on both its sides. Turn the brush over to transfer the resin from both sides of the brush to the surface.
As the resin soaks in, notice how the cloth has become visible first around cracks and staple holes. The cloth works as a wick, effectively feeding resin into the voids.
The surface of each plank will absorb the epoxy resin at different rates. It is important to supply enough resin to give each plank as much as it wants. Watch for dark heartwood planks, as they seem to absorb more than light-coloured planks.
WHERE TO BEGIN
When deciding where to begin applying the resin, the main consideration is keeping the edge of the resin fresh as you proceed down the hull wetting out the cloth. This will depend on the size of the hull and the number of people involved.
If you are working with just one person applying the resin, begin at the keel line about 3’ back from the end. Work down to the sheer and out to the end. This will tighten up the cloth and move the excess cloth out to the end of the hull.
Repeat on the opposite side of the hull, then move back to the side you began on for the third section. Change sides with each section of about 2’ to 3’ as you work down the hull.
When the first section has been wetted out, use this section as an anchor; working away from it to avoid bunching up the cloth.
By the time you get to about the halfway point, or about 20 minutes after starting, the area first wetted out will be ready for squeegeeing. It is important to squeegee only the first area, as the section beside it will have had less time to soak in – and thus not be ready yet for squeegeeing.
Keep track of the time. Someone came up with the "Time Tapes" trick to help his students. A piece of masking tape with the time that the resin was applied, plus 20 minutes is stuck on the strongback below the edge of the section. This tape indicates the time that section should be squeegeed. A good job for the mixer could be placing and monitoring the Time Tapes.
This might be the trickiest step of the whole building process, and it governs what your boat will look like. It is one of those steps that is very difficult to reverse. You need to be able to do it right the first time. I say that not to scare you – it’s not a difficult step – but rather that you give it your full attention.
The purpose of the squeegee is to even out the film thickness by scraping off the excess resin. The planking has now accepted all the resin it wants, the cloth is saturated but floating in places. Squeegeeing will leave the cloth evenly saturated and lying flat on the hull.
Most plastic squeegees have a slight curve along the edge. Hold it so that the corners turn up. Turned over, the corners will dig in and scrape too much resin out of the cloth. Hold it so that your thumb is on the bottom side and your fingers are spread out to control the pressure along the edge.
The angle that the squeegee is presented to the hull is most critical. The angle should be quite flat, with your thumb almost dragging on the hull. A steeper angle will put too much direct pressure on the cloth, and too much resin will be scraped out. Be conscious of keeping this low angle as you go around the turn of the bilge. There is a tendency to allow the angle to get steeper as you come around the turn of the bilge.
If too much pressure is applied, too much of the resin will be squeezed out of the cloth. This is undesirable, as after the resin sets up, the fibers of the cloth will be coated and sealed but not filled. The second coat will not be able to penetrate the fibers any farther and the glass fibers will remain visible; this will be most noticeable in bright sunlight on dark wood.
If you don’t press hard enough, the surface will remain shiny and the cloth will be floating. In practice it is a good idea to work a section down in stages rather than trying to get it all in one pass.
Begin at the centerline (keel line) of the hull and draw the squeegee down to the sheer with overlapping strokes. The speed should be slow enough to allow the resin to roll up in front of the squeegee.
The amount by which the successive passes overlap will depend on how much resin is being removed. If there is a lot of resin, pick up only as much as you can hold on the squeegee, and transfer it to the grunge can. As you get closer to the desired film thickness, use the full width of the squeegee.
Work gently over the section until the vertical shiny tracks are removed. In practice, you won’t get all of them, but try to get as even a texture as possible without wasting too much time. Keep in mind that this is practice for getting an even texture on the inside, where it will be more visible.
As you work these last tracks out, be very conscious of not taking too much and starving the cloth. The cloth will look glittery rather than wet-shiny if too much resin as been removed. Should this happen, add fresh resin rather than dragging old resin over to it.
FINISHING THE CLOTH AT THE STEMS
Wet the cloth out on both sides, then trim it to about 1" past the end of the stem. Check that the cloth is lying down flat to the hull along the edge. After the resin has firmed up (about three hours), trim it off clean with a sharp knife and soften the edges with 120-grit sandpaper.
THE SECOND COAT
The purpose of the second coat is to fill the weave and level the surface of the cloth. No attempt should be made at this point to build up the resin thickness. The surface is too coarse to brush on without the resin foaming up, and there is danger of air being trapped at the bottom of the weave.
The minimum time between coats is about three hours, or when you are able to trim the cloth without the resin sticking to the knife. The three coats should go on as close together as possible. Applying the second and third coats before the previous coat has cured will result in a true chemical bond.
If you can time it right (about four to five hours), a quick hand-sanding with 120-grit will take the tooth off and make the surface easier to work on. Since the depth of the weave is being reduced, slightly less resin will be required to level the weave.
If it has been more than about eight hours since the last coat was applied, the surface should definitely be sanded. This is important because the integrity of the bond between the layers now depends on the layers being mechanically, rather than chemically, bonded together.
On a horizontal surface or on the inside, it is possible to pour a small puddle of resin on the surface, then spread it around with the squeegee. The technique that works for me on the outside is to use the brush to transfer the resin from the container to the hull. Since the brush is depositing more resin than is necessary to fill the weave, only spread the resin about one-third of the way down from the centerline. This should be just enough resin to fill the weave all the way down to the sheerline.
The first time over, concentrate more on filling the weave than making it look good. Work the resin aggressively enough to force the trapped air out of the craters in the weave.
After roughly spreading each batch, go back over the section systematically, scraping off and disposing the excess resin.
Use a steeper angle and slightly more pressure on the squeegee this time to force the air out and to level the surface.
The part that is most likely to get missed is the vertical surface along the sheerline. Check for dull patches that would indicate an area that has been missed. It is hard to see on the vertical surface, so check from all angles.
Listen to the squeegee. As it passes over a dry patch you will hear a harsher sound.
If there is a problem getting the vertical tracks out, try working in a fore-and-aft direction. On the inside, where you want to get rid of all the tracks, go over the surface with a dry brush. This may be a reasonably aggressive motion in a fore-and-aft direction.
Check that the ends of the stems have been coated evenly.
APPLYING THE THIRD COAT
The purpose of the third coat is to bury the ‘glass cloth with enough epoxy resin that the surface may be sanded smooth without hitting the cloth.
Even though the surface has been squeegeed and the cloth is buried, expect to see a slight profile of the weave after the third coat.
This coat will be applied with a familiar painting motion. Since you are now working on a reasonably smooth substrate, a more aggressive motion may be used without introducing air into the resin. If you do see bubbles forming excessively, be less aggressive.
The idea is to apply as much resin as you can get on in an even film thickness without it running. You will have to use your judgment to find this balance between just right and too much. Too much will give the brush a mushy, skidding feel, while too little will have noticeably more resistance. Controlling the film thickness by feel is often more reliable than doing it visually, especially if the light is marginal.
Load the brush and apply resin to a section about 3’ long or as long a section as you can work comfortably. Spread it with a long, vigorous, but controlled back-and-forth motion to work the resin out into an even film thickness. Check for even film thickness by brushing up and down, feeling for consistent resistance through the brush.
After spreading the resin over the section being worked, go back to where you started and finish off with long, flowing, fore-and-aft strokes. This should help the resin blend in evenly with the adjoining sections.
When applying resin beside a completed section, work from the wet to the dry to keep the resin from building up too thick along the overlap.
There is a limit to how long you can play with working the runs out. At some point, the resin will begin to firm up and will pile up and now flow out. This is the time to quit, because from this point on working it will only make it worse. Don’t take a few runs on this coat too personally, as they happen to all of us. Remove them with a sharp cabinet scraper after the resin has cured.
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