A Guide to Sheathing
Contents
1. A GUIDE TO SHEATHING BRAND INTRODUCTION Why is it necessary to use WEST SYSTEM epoxy and glass cloth to sheath a structure This is a question we are frequently asked sometimes on a daily basis We explain that when a hull deck or other structural part requires stiffening strengthening or added resistance to wear then an epoxy glass sheath is recommended Furthermore by using glass fabric with epoxy the thickness of the epoxy glass applied is at least double that of the epoxy alone thus ensuring a superior protective barrier and enhanced resistance to moisture increase Another benefit of using glass cloth with epoxy is a degree of flexibility is introduced into the coating thus enhancing the impact and abrasion resistance of the epoxy glass membrane Examples of sheathing i To reinforce a GRP hull either damaged or suffering from a severe osmotic problem ii To stiffen and strengthen a wooden hull or deck iii To provide better impact and abrasion resistance to a structure e g below the waterline or on centreboards and rudders Having explained the reasons for sheathing the next questions are normally How do apply the glass fabric and Is this something can do So regularly do these questions occur it was decided to prepare this short publication to illustrate how simple the operation is and most importantly how to select the right materials in the correct quantities Glass Fabrics us2. cardboard or plastic tubes Position fabric if necessary using either staples tacks or tape to temporarily hold the cloth in position ii Mix enough epoxy to wet out the fabric On flat surfaces pour epoxy directly onto the fabric in small puddles and work epoxy into the fabric using plastic squeegees and or foam rollers to removed trapped air Caution Do not over work the fabric as this will disturb the weave of the cloth Do not use excess resin hardener as the fabric will float on the epoxy i e the glass cloth must be in contact with the surface iii Ensure the cloth is thoroughly wetted out Do not leave any white spots these are areas of dry fabric iv When the cloth has a dull semi matt finish it is wetted out If the surface appears shiny too much epoxy has been used v Move onto the next piece of fabric either butting or overlapping the fabric If using WEST SYSTEM 744 280 g m Twill cloth this fabric has tapered edges producing clean overlapped joints NOTE If working on uncoated wooden surfaces ensure the timber is warmed before application of epoxy Remove the heat source before applying the epoxy This will eliminate the out gassing produced when expanding air is released from the wood fibres If the surface is to be clear finished use lighter weight fabrics Although one layer of 280 g m Twill fabric will become transparent when wetted out the weave of the cloth may still be visible W
3. will ensure the Peel Ply is more easily removed Version A 05 04 Page 5 A GUIDE TO SHEATHING Overcoating Sheathing If not using Peel Ply the last layer of epoxy glass should be either i Overcoated with one or two coats of epoxy to fill the weave of the fabric but only when the epoxy glass laminate is touch dry Or ii Filled with one application of WEST SYSTEM epoxy 407 Low Density Filler By completing either one of the above when the epoxy is washed and sanded prior to overcoating the sanding ought not to cut through to the epoxy glass laminate If applying epoxy filler once cured and sanded two further coats of epoxy should to be applied Please refer to the WEST SYSTEM User Manual and Product Catalogue for further information Version A 05 04 Page 6
4. 0g 407 360g 105 205 Version A 05 04 Page 1 Selecting the Correct Fabric There is a wide range of fabrics available in the market place Within the WEST SYSTEM product range we have included the most widely used and easy to handle fabrics Given below are details of the glass cloths and their potential uses Style Weight Construction Uses Clear Laminate 736 300 g m 45 Biaxial General sheathing will add some structural strength Almost 738 610 g m 45 Biaxial General sheathing applications structural fabric No 739 450 g m 45 Biaxial General sheathing applications structural fabric No 740 135 g m Plain Weave Lightweight fabric to provide general sheathing Yes 141 200 g m Twill Weave General purpose fabric Yes 742 200 g m Plain Weave General purpose fabric Yes 143 280 g m Twill Weave General purpose will add some structural strength Almost 745 190 g m Crowsfoot Ideal for sheathing centreboards and rudders Yes 746 260 g m Woven Roving General purpose fabric Almost Why use Epoxy A specially formulated marine grade epoxy offers many advantages over both base epoxy products and polyester resins fibreglass resin A formulated marine epoxy will tenaciously bond to the substrate whether itis wood GRP or metal and will provide superior bond strength and toughness Low cost base epoxies and polyester resins do not have the same ability to resist moisture ingress and may in time crack and peel from the underlying surface The ch
5. ed in sheathing operations Biaxial two layers of unidirectional fabrics stitched together and 45 to the O plane gives the orientation of the two layers Plain Weave woven glass fabric strands of fibre are woven in a square pattern one set of fibres in O plane the other in 90 plane fibre strands woven one under and one over Twill Weave Woven glass fabric as plain weave but fibres are woven two or three under one over Crowsfoot Weave The 745 fabric has 80 of the fibres in the 90 plane and 20 in the O plane The woven fabric is easier to use as it will wet out more readily the biaxial fabric is more heavy duty and requires more work to effectively wet out the fibres We do not list Chopped Strand Matt CSM as this type of glass is designed for use with polyester resin and provides little benefit when used with epoxy Estimating Quantities Required When applying sheathing it is important to correctly estimate the material required i Allow 15 to 20 more in addition to the surface area for the glass quantity ii Use the following estimates for calculating the quantity of epoxy required Coating 135g of epoxy to coat 1m Sheathing Allow the same weight of epoxy as the fabric ie if the cloth weighs 300 g m Allow 300g of epoxy per m allow approximately 15 extra for wastage Fairing 600g mix of WEST SYSTEM epoxy 407 Low Density Filler will provide 1mm thickness over 1m 600g mix 24
6. emove the overlapped trimming d Re wet the underside of the raised edge with epoxy and smooth into place The result should be a near perfect butt joint eliminating double cloth thickness The disadvantage of joining fabric in this way is that the knife will mark wood and this is not suitable when a clean finish is required However a lapped joint is stronger than a butt joint so if appearance is not important it may well be advisable to leave the overlap and fair in the unevenness after coating Any remaining irregularities or transitions between cloth and substrate should be faired by using an epoxy filler fairing compound if the surface is to be painted Any fairing completed after the final glass cloth layer should receive several additional coats of epoxy over the faired area once surface preparation has been completed If using WEST SYSTEM 743 280 g m Twill fabric a perfect joint is achieved as this cloth has tapered edges When a clear finish is desired this fabric is a good option Using Peel Ply The use of peel ply in the sheathing process is extremely important The closely woven nylon cloth is a release fabric to which epoxy will not adhere and is applied as the last layer of the sheathing process The Peel Ply fabric is thoroughly wetted out and must be in full contact with the underlying laminate Why use peel ply These are the major advantages of using the cloth i The epoxy glass sheathing is evenly consolidated as mo
7. et Method Once the surface is prepared and the glass cloth cut to size i Roll a heavy coat of epoxy onto the surface ii Carefully unroll the glass cloth into position over the wet epoxy Surface tension will hold the fabric in position if applying to vertical or overhead surfaces allow the epoxy to become a little tacky say after 30 to 45 minutes as this will help to hold the fabric in position Work out wrinkles and excess air using either a plastic squeegee or foam roller iii Re apply epoxy to any dry areas iv Squeegee out any excess epoxy v Once fabric is wetted out move onto next section When using the wet method it may be advantageous to cut the fabric once wetted out This will ensure accurate placement of the fabrics and stability of the cloth from the roll Use an adapted clothes rail taught rope or a broom handle to allow free dispensing of the cloth from the roll Clean scissors etc with WEST SYSTEM 850 cleaning solvent Version A 05 04 Page 4 A GUIDE TO SHEATHING WEST SYS TEM BRAND Joining Fabric Trim the excess and overlapped cloth after the epoxy has reached its initial cure The cloth will cut easily with a sharp utility knife Trim overlapped cloth as follows a Place a metal straight edge on top of and midway between the two overlapped edges b Cut through both layers of cloth with a sharp utility knife c Remove the top most trimming and then lift the opposite cut edge to r
8. ntain a temperature of between 18 to 25 C Store the products in a warm dry area Warm the surfaces to be coated to a similar temperature to the workshop to aid in applying the epoxy and when coating wood this will eliminate any out gassing from the timber O AUN Page 2 Surface Preparation Always ensure surfaces to be coated are clean dry and thoroughly abraded 80 grit paper i Oily Timber In addition to the above degrease oily timbers such as teak using a volatile solvent for example pure acetone Furthermore sand across the grain to thoroughly roughen the timber face of hardwoods ii Metal The minimum requirement is to degrease abrade back to bright metal and degrease again Either grit or sandblasting is the best method for surface preparation Apply first coat of epoxy within 2 to 3 hours after second degrease to avoid oxidation of the metal surface iii Metal Alloys The preparation given above in ii is the minimum requirement however it is recommended that alloys are chemically etched in order to provide the best adhesion Seek advice from WRA Filling amp Fairing Fill amp fair the surface to be sheathed using WEST SYSTEM epoxy and 407 Low Density Filler mixed to a peanut butter consistency Applying an epoxy fillet to any external angles will allow the fabric to be more easily applied When the epoxy filler has hardened sand fair before application of the epoxy glass sheath We would recommend appl
9. oice of resin is crucial selecting a formulated epoxy product will in the long term reduce costs and maintenance As a general rule WEST SYSTEM epoxy will be used in most sheathing operations However if the area to be sheathed is large or a number of heavy weight multi axial fabrics are used the PRO SET laminating epoxy should be considered For applying light weight fabrics WEST SYSTEM 105 Resin with 205 Hardener can be used to a working temperature of around 20 22 C Above this temperature or if a large area is to be sheathed use 206 Slow Hardener If you require assistance in selecting the correct hardener please contact our Technical Support Group Working Conditions Environment It is important to ensure the correct working conditions are maintained when sheathing At warmer temperatures the resin and hardeners are more easily dispensed more readily mixed and much easier to apply to the substrate In addition due to the lower viscosity of the epoxy at warmer temperatures the fabric is easier to wet out Working indoors with adequate heating is the ideal situation but unfortunately this is not always possible Erecting a tent around the boat and providing warm dry heat will be adequate Storage of the resin and hardener is important Keeping both components in a warm dry area or using a hot box drawing available to store the epoxy will ensure the components are easy to use Work undercover Provide warm dry heat Try to mai
10. re pressure can be applied without disturbing the fabric ii Excess epoxy is brought to the surface and is removed from the laminate iii The epoxy glass surface is protected and any amine blush will occur on top of the peel ply Furthermore the Peel Ply will allow walking over the sheathed area as the Peel Ply will protect the underlying epoxy glass sheathing iv When the Peel Ply is removed the textured surface will not need sanding thus allowing further layers of glass epoxy fairing compounds or coatings to be applied Only remove Peel Ply just before the application of subsequent coating v Overlap joints and stitching on multiaxial fabric are feathered out by using Peel Ply The additional cost of using peel ply is far outweighed by the reduced labour involved in sanding a cured epoxy coated surface The Peel Ply can remain on the surface for a long period of time as it will still be easily removed thus protecting the epoxy glass sheathing Peel Ply is applied as the last layer in the sheathing process or the last layer applied in a single day Layout the fabric on the epoxy glass surface Wet out the peel ply using foam rollers and or squeegees There will probably be enough excess epoxy in the laminate to thoroughly wet out the fabric however if there are any dry areas wet out by applying more epoxy Overlap the peel ply where necessary Important Ensure there is an un wetted edge of Peel Ply When cured this dry edge
11. ying peel ply tape over the fillet joint as this would eliminate the need to sand before applying further epoxy Trowel epoxy mixed with 407 Low density Filler to peanut butter Consistency into voids Applying an epoxy fillet to any external angles will allow the fabric to be easily laid Page 3 A GUIDE TO SHEATHING Applying Glass Cloth There are two methods for sheathing the dry and wet method There is a variation to these methods wetting out the fabric either on a flat polythene covered table or by using our impregnator rolling the wet fabric on a plastic pipe and then laying out the fabric onto the surface Squeegees are then used to remove air and any excess epoxy from underneath This variation is particularly useful when sheathing large areas The Dry method For best results the surface must be pre coated with one or two coats of epoxy allowed to cure and sanded 60 100 grit This will reduce the problems associated with out gassing under the fabric by sealing a porous material such as wood If sheathing the hull the surface would be coated and faired prior to applying the fabric The dry method is ideally suited to sheathing flat areas decks etc or inverted hulls Generally the dry method would only be used with lighter weight fabrics up to 300 g m Above this weight or when using multiaxial fabrics it is difficult to wet through the cloth i Cut fabric to correct lengths and roll onto
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