Fiberglass and gelcoat repair applications

When fiberglass laminating be sure you have laminating resin not finish resin as finish resin has wax additive and must be sanded if top coated.  Finish resin is rarely used in boat hull repairs or boat building.

Stress cracks should always be dish ground and fiberglass laminated using 2 or 3 strips of biaxial cloth with matt sewed on the back.  Grind deeper wider and apply multiple strips if repairing corner rounds followed by specified filler, the same procedure as repairing a deep gouge as shown in the instructional using at least a 3″ grinding disc.  You may just grind the gelcoat off then a fill and fare but most likely the cracks will return quickly.  Simply grinding a gelcoat stress crack using a dremel grinder then a fill even with epoxy will crack sooner than later.  A professional should check stress cracks below a waterline.

If filling gelcoat chips or voids deeper than 1/8 inch, finely chop fiberglass cloth and add it to activated fiberglass laminating resin for base filler saturating until white air bubbles are gone and then continue with your gelcoat repair.

Repairing a crack thru fiberglass make sure to dish grind 5 to 6 inches each side of the crack, fiberglass laminate and repeat on backside for maximum strength.  Be sure when grinding on backside to meet the new fiberglass in along the center removing all fractured glass.

For fiberglass boat repair use 18 oz. cloth or larger up to a very heavy duty 24 oz. woven roving for the major repairs.  Biaxial with 3/4 oz. matt sewed on the back is an ideal cloth for all types of laminate work and forms well around corners.  If fiberglass laminating a large area, follow the same procedure as shown laminating a deep gouge in the instructional, all size damage is treated equal.

If replacing a rotted deck core:  Do not use the old fiberglass skin unless you do not care about voids, waves and surface cracks forming within the first few weeks.  When removing the old skin, fractures form in the layers of fiberglass and the gelcoat will fracture throughout the surface that is unvisible to the eye when removing, after the skins edges have been fiberglass laminated fared in and top coated, the final finish will only be a bridge over the fractures and all fractures will re-appear sooner than later, also you risk the possibility of voids beneath the skin when attaching to the new wood core.  If using the old skin be sure to grind the entire surface to apply at least one heavy sheet of cloth to cover all hidden fiberglass fractures.

When repairing snap holes or any small drill holes, simply filling the hole with polyester filler, auto bondo or gelcoat filler paste will definatly crack around the edge sooner than later.  Clean the hole using a drill then use a chamfering tool to slightly clean up any broken edge, then chop up fiberglass cloth very fine, saturate with polyester resin and stuff hole, when hardened dremel grind just the thickness of the existing gelcoat then continue with your gelcoat repairs.

Epoxy will adhere to polyester but polyester will not adhere to epoxy.

If repairing fiberglass damage on wave runners make sure to use epoxy resin with fiberglass cloth as polyester resins will not adhere to those types of composites.

Using polyester resin for filler mixed with Cabosil, Aerosil or Cotton for mounting any structure even filling delamination in a deck can get very hot if thick and may warp fiberglass do to extreme heat.

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About the Author

John Gabriel has experience with fiberglass and gelcoat repairs and the science behind it for over 30 years and now shares the knowlege worldwide to all boat owners that have interest.

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