Fiberglass Boat Molding Tutorial

Fiberglass Boat Molding TutorialMost of today’s boats are built using fiberglass as the main material and molding as the method of building. Usualy there is a huge negative mold for every boat part that used when the boat is put together and this negative mold is used to make positive molds. Boat is then assembled form a bunch of parts that are made from molds. Molds are also made from fiberglass and are covered with a perfect layer of molding gelcoat. Boat builder than applies some molding wax over the glecoat and polishes it so it will later on be easier to remove the part form the mold. Molds are usualy build on a large scale part that is sanded and polished to perfection. Parts are usualy made out of foam or wood. Boat hulls are typically made from plywood or some other wood.

You may also want to read the Fiberglass Guide before you start working on your mold or find an appropriate boat plan to build mold form.
Be sure to check the Boat Building Supply Shop for all you need for boatbuilding.

The Plug

If you want ot build a mold you must have a form from which you will develop your mold. You must either build a part from scratch using wood, plaster, polyester putty, sheet metal, etc. or you must have a completed part which you wish to duplicate. The plug is generally a male model (a positive model) exactly like the item you wish to build in all the details. If the plug has reverse bends, like many canoes, then you will need to make a split mold which can be spread or taken apart.

To prevent your mold from sticking to the plug, the plug must be coated with plastic film known as “PVA”. This is a plastic dissolved in alcohol and has a green color. It can be brushed or sprayed on, but the best system is to spray on three thin coats, the first being a “mist coat”.  The appearance will then be green. Each coat must dry half an hour or so and there must be no pools or drips to blemish your mold surface. For the easiest possible parting, before applying the PVA, apply a soft wax formulated for use with PVA. After the third coat of PVA has dried, a coating of this wax can be gently applied over it for easy parting.

The Mold

The first step is to apply gelcoat which will be the mold surface. The gelcoat must be “exterior gelcoat” type (wax free). If many parts are to be taken off the mold, it is desirable to use a “tooling gelcoat” which is designed to give longer life in mold use. The gelcoat should be in contrasting color to the surface of the part you will make. Since most parts are light colored, black gelcoat is commonly used. This facilitates spraying up a uniform thickness of light colored gelcoat since the black will show through thin spots.
When the gelcoat has cured so that it cannot be scratched off with the fingernail at the edge of the mold, which takes from 2 to 4 hours to overnight in cool or humid weather, you are ready for the “skin coat.” This is a layer of ¾ or 1oz. fiberglass mat, thin enough so you can see and remove all air bubbles entrapped by the resin when you “wet-out” the mat. The resin should be applied with a mohair roller or brush until no white fibers remain. Any air bubbles are then eliminated with a grooved plastic or metal laminating roller. The polyester resin used should be “lay-up resin,” which is wax-free. Be careful not to over-catalyze when laying up the glass.

Removing Mold From The Plug

Allow mold to cure several days if possible so it will hold its shape. The first step is to trim the excess laminate back to the molded edge. This is easily done with a saber saw and a metal-cutting blade. The the edges are sanded carefully until the line between the mold and plug is exposed. Then a sharpened “tongue stick” is forced between mold and plug to separate the edges. The stick is then pulled clear around the plug until all edges are free and no bridges remain. Avoid using metal tools for this purpose as they will scratch the mold surface. Then the mold should pull free of the plug. If not, the parts can be flexed or pounded gently with a rubber mallet. If necessary, air or water can be forced under pressure between plug and mold. A hole can be drilled through the interface for this purpose. PVA is water soluble, which facilitates parting with water pressure.

Polishing The Mold

Depending upon the condition of the mold surface, it may have to be sanded with 220 grit working up to 600 grit wet or dry. The surface is then compounded with regular and fine finish compound formulated for fiberglass work. Best results can be achieved by using special compounds such as our “Heavy Duty Cleaner” followed by our “Sealer Glaze” to bring out a mirror finish.

Article source: Making a Fiberglass mold

Check this great video about vacuum molding and boatbuilding

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