How to Fiberglass a plywood boat - Glen-L Boat Plans

How to Fiberglass a plywood boat

If you have not worked with GLEN-L Poxy-Shield epoxy resins, or you have applied fiberglass before using polyester resins, make sure you read and understand all of the following information before starting any aspect of the job or handling any materials. Epoxy resin works and handles differently from polyester. SAFETY PRECAUTIONS ARE IMPORTANT!

The Poxy-Shield epoxy resin consists of 2 PARTS. These are to be mixed according to the ratios given on the containers; 5 parts A (resin) to 1 part B (hardener). DO NOT VARY THE RATIOS. Do not add solvents or thinners. Sanding between coats of resin is not absolutely necessary to achieve a bond (although some sanding may be required as will be explained later). Note: Resin without hardener is NEVER used. Always add hardener in the 5 to 1 ratio.

The fiberglass cloth used in fiberglassing a typical plywood boat must be compatible with epoxy resin. Although most cloths will work with epoxy, there are some that are not compatible. If in doubt, test the material using scrap wood, to insure that it cannot be pulled off. Fiberglass mat should not be substituted for cloth. The cloth included in Glen-L Fiberglass Covering Kits are, of course, compatible. Glen-L kits include a cutting list to determine how to cut and use the cloth provided. Cloth comes in various widths and are chosen to yield as little waste as possible.

Resin is applied using disposable brushes and foam rollers. You will also need a squeege for working out excess resin when applying the bond quote. Power sanders will save work during finishing and other sanding operations. The reciprocal/orbital types are safe and easy to use, but are slow and remove only small amounts of material. The disc and belt types are fast, but require some practice for proper control. A foam pad backing disc should be used to minimize gouges. A sanding block can be used in areas where power tools can’t be used. Scissors and a utility knife are used to trim the fiberglass material. Cleaning materials should be ready to use at the work site. These would include a bucket of hot soapy water or a waterless resin cleaner, safe for skin contact; and suitable solvents, such as denatured alcohol, acetone, or lacquer thinner, for keeping tools clean.


  • Do NOT work in direct sunlight or where there is direct exposure to the elements. Do the job under some sort of cover that will prevent any moisture, dirt, or debris from settling onto the surface.
  • Do the application at temperatures between 70oF and 85oF for best results. Temperatures below this will retard or prevent cure, while temperatures above this may cut working time to an impractical degree.
  • The area must be well-ventilated, but free of wind or high-speed fan-forced air.
  • Start applying coats of resin early in the day so that partial cure will occur before any dew, moisture, fog, or rain can settle onto an uncured surface.
  • Do NOT work in high humidity conditions or when rain is imminent or in progress.

Cleanliness is the most important factor for successful results. The application and all coatings should occur over a clean, dry, bare wood surface free from dirt, dust, oil, grease, wax, paint or other contaminants. The use of wood preservatives on the surface to be covered is not recommended. A slightly rough surface is acceptable. Avoid over-sanding Douglas fir plywood surfaces as the softer grain will wear away, creating a wrippled surface. Should this occur, correct after fiberglassing, using a filler such as our Microspheres. All holes, seams, cracks, dents, gouges, and other imperfections can be filled with a non-oily resin-compatible wood putty or epoxy filler, and sanded level prior to applying the first coat. Optionally, this work can be done after the seal coat has cured but in this case, only an epoxy filler can be used. Do NOT use polyester-based putties. Appendages, such as deadwood, keels, lift strakes, and rub rails should be installed AFTER the fiberglass application. All corners (inside and outside) should be well radiused. See note in “Finishing” for more on the radiused edge of the transom.

Read all product container labels before opening. Epoxy resins are considered industrial chemicals that should not be handled carelessly. AVOID DIRECT SKIN OR EYE CONTACT. ALWAYS wear protective gloves, eye protection, and non-permeable protective clothing (such as paper or plastic aprons). ALWAYS wear a dust mask when sanding, especially on resin-coated surfaces that have cured LESS than 7 days.

When the RESIN (PART A) and HARDENER (PART B) are mixed together, a heat-producing reaction (called “exotherm”) begins. No reaction will be noticeable for about 30 minutes at room temperature, but after this time, the mixture will thicken and change to a solid material. The rate of this change can vary. Cool temperatures slow down the reaction; warmer temperatures speed it up. Also, large amounts in a concentrated area will cure faster than a comparable amount well-dispersed over a larger, flatter area. (As a precaution, large concentrated amounts should be avoided since the heat can build up to such an extent as to present a fire hazard.) The worker can control the “exothermic” reaction to some extent. For example, slow the reaction down by placing the contents in a cool area or in the shade; some even place the container in an ice box (free from foodstuffs). Or in cooler temperatures somewhat below the 70oF level, the product containers can be brought up to room temperature by placing them in WARM (not hot) water before mixing. It is also possible to bring up the work surface temperature with locally applied heat, such as with light bulbs. However, do NOT attempt to change the reaction by varying the mix ratio, and do NOT attempt to use thinners or solvents in cold weather to make the products easier to mix; bring them up to room temperature and mixing will not be a problem. Keep containers closed when not in use. Store at room temperature and avoid exposure to heat and freezing/thawing cycles. If crystals form in products, set containers in warm water; they should disappear and no harm is done to the product. If products stand for more than 6 months, stir or agitate before use. Follow mixing instruction on the container labels. Do NOT mix up more than about a quart at a time, and smaller amounts are usually preferable. Mix up only as much resin as can be worked in a 20-30 minute period. Dispense into a shallow, flat pan or paint tray IMMEDIATELY after mixing. (A possible exception to this is that in temperatures cooler than recommended, the mixture can remain in the mixing container for a few minutes to help get the reaction going prior to dispensing.) Do NOT mix in glass or foam plastic containers. A CLEAN stick of wood is suitable for mixing. Certain power-type mixers can be dangerous and make mixing incomplete. Do NOT use the mixed resin once it begins to gel or gets “stringy” discard it and mix up a fresh batch. Resin is ALWAYS used with hardener, there is no application where resin is used without hardener.

The application consists of four coats of resin. the FIRST coat is the SEAL COAT, and seals the wood surface prior to applying the cloth. The SECOND coat is the BOND COAT, and is used to wet out and bond the cloth to the surface. The THIRD coat is the FILL COAT and fills the weave of the cloth. The FOURTH coat is the FINISH COAT and provides enough resin build-up for final sanding and finishing. Throughout ALL coats, do NOT apply any more resin than is necessary to accomplish the purpose of each coat. A typical problem with beginners is that they use too much resin. This is a waste of resin, makes finish work difficult, and increases weight and cost. Do NOT attempt to spray any of the coats of resin.

After surface preparation, start early and apply the first coat of resin to all surfaces that will later be sheathed. Use a thin high-density foam roller with long multi-directional strokes and good pressure. This coat should be thin and even, free from runs, sags, high and low areas, and “dry” spots. After 10-15 minutes, check the surface for any dry spots where the wood has absorbed the resin (especially over edge or end-grain areas), and recoat. Air escaping from the wood may cause small bubbles to appear. This is a proper occurrence and indicates proper sealing; too thick a coat will inhibit this reaction. Work the coat as long as possible to assure that all areas are evenly coated. Work from a dryer area to a wetter area; there is no need to stop. However, don’t recoat an area if the surface has begun to set up. Allow this coat to cure at least tack-free before doing subsequent work on the surface, or overnight. The surface may appear fuzzy upon cure which is usually of no consequence. However, if runs or sags or extreme roughness occurs in an area of a type that would interfere with the easy and smooth application of the cloth, these areas should be sanded. However, heavy power sanding on this coat should be avoided to prevent removing the seal coat or damaging the wood surface. A light hand sanding and solvent wipe should be all that is necessary prior to the Continue coat (*).

(*) Sanding after each coat is not necessary for adhesion if the coat is applied in less than 24 hours after the previous coat. However, a light sanding is often advisable to remove any dust, dirt, insects, or debris that could have settled onto the surface during cure. In addition, a solvent wipe after sanding using a rag soaked with denatured alcohol, acetone, lacquer thinner, or similar solvent will remove any loose dust and other contaminants. With all cured coats of resin, a oily-like film (amine blush) frequently appears on the surface. This thin film is easily removed with solvents as above, if left on the surface it can cause bond problems. Make sure the surface is completely dry before recoating.

We recommend the “dry method” of cloth application. In this method, the cloth is positioned over the surface “dry” and the resin applied ONTO the cloth to wet it out.
Cut, fit, and position the fiberglass cloth first, using the cutting instructions provided. If you are not familar with working with fiberglass cloth, start the application in a smaller area, such as the transom, in order to get a “feel” for the work. Use tacks, staples, or masking tape to hold the cloth in position, but make sure tacks and staples are removed prior to the resin setting up hard. (We use masking tape, applied to no more than an 1/8″ of the cloth. Fold under the end of the tape that sticks to the hull, so it can easily be removed.) Begin work from one end of the hull and work to the other; once application of the bond coat begins, there is no need to stop (except in certain instances as will be noted).

Joints in the cloth are usually required. Double laps of cloth may be specified, such as at transom corners, chine junctions, across the centerline, etc. This is to provide a double thickness of material at these areas. At these junctions, apply the cloth to lap across such corners and junctions about 4″ or as specified. Allow the resin to cure tack-free, then feather the edge of the cloth a couple of inches by sanding. The adjoining cloth then laps over this area covering the feathered area and preventing the formation of a ridge at the cloth edge. Feathering again will be done on the second (adjoining) cloth after the resin cures, making such joints inconspicuous. At other joints which occur between widths of cloth, these joints can simply be buttted (although overlaps are optional and would be handled similarly as noted if done). Attempting to fit or butt cloth widths together closely can be difficult.

Apply activated resin onto limited area of cloth and move it around quickly with a roller, brush or squeegee. Apply resin to upper areas and work to lower areas. After all material is wetted, work out excess resin with a squeegee, being sure that there are no “resin bubbles” under the cloth. Keep the squeegee constantly in motion using tight-S strokes. Work from wet areas to dry areas, keeping resin ahead of the squeegee. Use firm pressure to force the resin into and through the cloth, but don’t drag the cloth from position. Don’t allow any dry (“white”) spots; apply more resin to these areas. When properly applied, the cloth will become transparent and develop a “matte” finish; the weave of the cloth should be obvious. Glossy areas indicate too much resin. If air bubbles develop, work these out with the squeegee. If air cannot be removed from an area and the resin is starting to set, cut the area with a knife and force the cloth down. Do NOT attempt to fill the weave of the cloth; use only as much resin as is required to bond it to the surface and wet it out. If there is too much resin on the surface and it begins to set up, remove it immediately and discard it. Work the resin as long as possible to prevent runs and sags, particularly on inclined or vertical surfaces. Trim excess cloth BEFORE the resin sets up hard, but not before the resin sets up enough to prevent dragging the cloth from position. There is no need to stop the application, however, if time runs out, trim the cloth at the stopping point to make a crisp edge. The work can proceed at a later time by butting the subsequent cloth to this edge in the manner noted previously. If, in spite of your best efforts, white spots (insufficient resin) are noted after the resin sets up, sand away to well-bonded material and apply a fiberglass patch.

No sanding of the surface is necessary before subsequent coats, and these can be applied as soon as the previous coat sets up tack-free if time allows. In fact, the sooner the better. However, if runs or sags occur, the resin should be allowed to cure so that these areas can be corrected by sanding. Just take care in all sanding operations NOT to cut through or into the weave of the cloth. If this occurs, the area must be patched. Furthermore, if a long period passes between coats (24 hours or more), the surface should be cleaned with a solvent wipe to remove any possible dirt, debris, amine blush or other contaminants before applying additional coats. The third or FILL COAT is used to conceal the weave of the cloth, and is best applied with a new foam roller as was done on the first coat. Other application tools will not make an even coating and will be more difficult to use. The surface will be slightly stippled when using the roller. To prevent this, a brush can be used to even out the coating before it sets up, either a bristle or foam type, after roller application. Again, don’t apply so much resin that runs and sags develop. If this tends to occur, it is better to apply two or more thin coats instead of a heavy single coat that will not stay put. Allow this coat to cure tack-free.
Note: When using the roller, do not vigorously run back and forth over an area to wet it out as this can “whip” air into the resin and cause excessive bubbling or foaming.

The final FINISH COAT is applied to provide a sanding and finishing base so that enough resin is available for sanding to prevent cutting into the cloth. This coat is applied in the same manner as the FILL COAT. Take care to assure that this coat is as smooth and even as possible, using a brush to smooth out any imperfections before the resin sets up. If the resin coating is not sufficiently thick at this point, apply additional coats as necessary, but remember, thinner coats are better than thicker ones. Allow the FINAL COAT to cure long enough so that sanding will be easier (overnight or 24 hour cure is minimum preferably.)
Note: If the boat you are building is a speed boat or other planing boat, the junction of the bottom and the transom should be a fairly crisp angle. Before fiberglassing, this edge was rounded for fiberglass application, now you must put the edge back. Use scraps of cloth and epoxy to build the corner up, then grind the material off to form an edge. Be sure that no “hook” is built in, as this, like a geneously radiused edge, can cause performance problems.

The finish work will vary depending on the quality of the FINISH COAT. If the work has been done with care, start out by sanding with an 60 to 80 grit paper, working up to a 120 to 180 grit. For many builders, this will give results good enough for painting. However, progressive sanding using finer grit papers will result in an even finer, smoother finish. Many approaches to final finishing are possible and are beyond the scope of these instructions. In any case, the final epoxy coated surface MUST be protected by a paint system or ultra-violet stabilized coating. These may or may not require primer undercoats. Just about any marine-type paint system or industrial enamel can be used; much depends on the builder’s budget. In all cases, follow the instructions provided by the paint manufacturer to the letter. Note: The resin rarely gives a varnish-like smooth surface. For natural finishes, sand the epoxy with a fine grit sandpaper and apply multiple coats of marine varnish or polyurethane. Also, for natural finishes, be aware that epoxy resins cause slight color changes in the wood. If coating wood and only part is coated with epoxy, then varnished, the surface can look “blotchy”.


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