Composite Plug and Mold Making Materials

Below is a list of materials that are used in plug and mold making when making fiberglass or composite molds or tooling and links to the catalog pages for the items we carry. If you are interested in learning more about the process we documented the process of making a longboard skateboard mold from scratch on this page. We periodically hold demonstrations on plug and mold building, you can find the schedule in the Demo page. If you have any question please feel free to contact us we’d love to help you with your project.

Last-a-foam, a urethane foam that is easily shaped and comes in various densities.  The higher the density the harder it is to shape but the better detail it will hold. Available in a variety of thicknesses.

MDF- is a commonly used product for plug making, it is relatively cheap, can be shaped relatively easily, has a low young’s modulus, and is heavy.  If moisture content is kept low it can be used at higher temperatures and has a low coefficient of thermal expansion.

Pour foam and spray foams: come in a variety of densities, we stock pour foam in 2lb and 8lb densities.  Can be a suitable way to get a base for a plug, for making repairs to a plug, or as part of a frame/skin plug construction.

Tooling Clay- Non hardening plasticine clay is often used to form fillets, fill holes, or gaps on plugs and tooling.  Use where you need a temporary putty or surface.  

Styrofoam- cheap, not super easy to shape and performs rather poorly at holding tight details.  Can be very lightweight and may be suitable for prototyping and some plug work.  Is not compatible with polyester or vinylester resins. Compatible with epoxy or specialty coatings only.

Body filler- commonly known as bondo. Cures quickly and can be filed, then sanded in a relatively short period of time.  Available in quart and gallon quantities.

Quick Fair- an epoxy based fairing compound with a very buttery texture, It has a longer working time than bondo and is suitable for sculpting fine details.  Cures to a sandable state in 3-4 hours at 77F and sands easily. Available in 24oz, 48oz, 96oz, and 1.5 gallon kits.

Filled resin- Putty made from resin, 3M bubbles, and cabosil can make a suitable material to build plugs out of. We carry those fillers.

Glass Fabric-  Lightweight fabrics (4oz and 6oz) are suitable for covering plugs to provide a solid surface to work from.  Fabrics generally will only accept gentle and simple curvatures, turning the fabric on the bias may help with some curvatures.  Often on very complex parts it helps to lightly spray glue the fabric in place before laminating. Available in 30, 50, and 60” widths (6oz only in 60” width)

Chopstrand mat- The surface of chopstrand will require more work to fair than a glass fabric however in a part that has very complex curvatures AND is compatible with polyester resins chopstrand may be the way to go. Chopstrand is only compatible with polyester or vinyl ester resins. Available in 3/4oz and 1.5oz in 38”, 50”, and 60” (3/4 oz only in 60”) widths.

UV Cure polyester resin-  Is a great way to get from step to step quickly.  Cures in under 10 minutes under UV light (sunlight).  Works best in thin layers.  Available in pints, quarts, gallons, five gallon pails, and drums.

Duratec Surfacing primer-  A high build primer that is great for applying over a prepared plug.  It is polyester based and NOT suitable for use directly over styrofoam. Can be applied up to 40mils (.040”) which is the thickness of 10 coats of regular paint.  Can be buffed.  For higher gloss application mix with Duratec High-gloss additive. Available in Quarts or Gallons

Duratec High Gloss Additive- Mix with Duratec Surfacing primer or gelcoat to make a sprayable high gloss coating.  Your mold surface will be no better than your plug surface so top coating with High Gloss Additive and gelcoat gives you a coating that can be buffed to a high quality finish. Available in Quarts and Gallons

Gelcoat- When topcoating mix with duratec high gloss additive to get a coating that will flow and cure to a glossy finish.

Gloss Resin- We carry a finishing resin that is typically used in surfboard building as a topcoat and is suitable for top coating plugs with.  It is a clear resin so typically you’ll want to add a pigment to give it color and make buffing and fairing more accurate.  Gloss resin can be applied with a brush and is self leveling.  Wetsand and buff for the best finish possible.


Sand paper and sanding blocks-  Rougher grits for shaping and fairing, finer grits for finishing.  We have a wide array of sanding blocks and boards including the 30” 3M fairing boards in rigid and flexible.  Sanding blocks help to get fair, flat surfaces.  For irregular shapes you can make your own sanding/shaping tools by spray gluing sand paper to your specially shaped backer board, high density foam can also be used for this.

High grit sand papers- for final finishing sand to 800 grit or better before compounding and polishing, we carry wet/dry paper up to 2000 grit.

Rubbing compounds and polishes- 3M Heavy Duty, 3M Finesse-it 2, and surfboard polishing compound.  Start with Heavy Duty and work to either the Finesse-it 2 or the surfboard polishing compound.  I usually use the surfboard polishing compound with a power buffer. Surfboard Polish is available in 4oz, 8oz, Pint, Quart, Gallon, and 5 gallon buckets.  3M Heavy Duty is available in pint, quart and gallons.  Finesse-it 2 is available in pints.

Mold Releases

For Plug work you want to stick with a release wax, possibly in combination with PVA

We have a variety of paste waxes, the two that I tend to use the most are Partall #2 when working with PVA, and Honey Wax.  We do have a lot of customers that prefer TR High-Temp as well.  You want to stick with a wax instead of a semi permanent because it isn’t as slick as the semi-permanent and the mold will hold better to the surface while it is being built, reducing the likelihood of pre-release.

PVA is vinyl in an alcohol solution that you spray over the plug and it dries into a film.  Follow the instructions and if in doubt do multiple coats to get a thicker film.  Most issues will come from either putting on too much in one coat and getting puddling or not getting a thick enough total film thickness and having the PVA stick to the mold or leave a textured surface. PVA is available in 2oz, 4oz, 8oz, 16oz, 32oz, gallon and 5 gallon sizes.

Semi-permanent waxes- Semi-permanent mold releases are great for production but are very slippery and not generally recommended for mold building.  Semi-permanents consist of a two step process, a sealer and the release wax.  The sealer reduces or eliminates issues related to surface porosity and is highly recommended especially on surfaces that have been sanded or are made from materials known to have porosity issues.  Semi-permanents are great for getting a mold into production quickly and for being able to release multiple parts from a mold.

Tooling Clay - non hardening plasticine clay that is used to fill minor imperfections or create fillets.

Gelcoats - Tooling gelcoat is a specially formulated gelcoat that is designed to be stable and take the rigors of part making.  It is not designed to be left out in the weather or have the same flexibility of a gelcoat used on the final part.  It does have a higher hardness to resist damage from the molding and demolding process but it is expected that the tool is has the rigidity needed to not flex too much.  Tooling gelcoat can be blended with Duratec High Gloss Additive to increase its properties.

Gelcoat needs to be applied at 20 mils thick and at the proper catalyst ratio to achieve cure in 45-60 minutes and kept at temperature so as to not retard the curing process.  Retardation of the process will result in a coating that does not have full strength characteristics. This applies both to tooling gelcoat and regular gelcoat.  The best method of application is spraying.

Catalyst- MEKP is recommended for both the gelcoat and the laminating resins used in mold making.  Other catalysts like AZOX have a higher likelihood of distorting the mold as it cures.  AZOX is NOT recommended for gelcoats, period.  AZOX has a higher water content which creates some issues with gelcoat.  For molded parts however AZOX can significantly speed up process times.

Iso tooling resin- Is a great resin for building molds and parts from, has higher strength characteristics than Ortho resins and more importantly for mold making has very little shrinkage.  

Vinyl Ester Resins- Has the highest strength of the styrenated resin systems and very low shrinkage as well.  Vinyl Esters also have higher heat deflection properties and can be used at high temperatures. A better choice for molds that will be heated or if working with carbon fiber.

Coring Materials- Molds generally should be stiff and rigid.  Coring may help with this.  Balsa core is commonly used in flat areas or areas with gentle curves.  Core mat, a laminate bulker is also commonly used.  Core materials also act as insulators and consideration needs to be given to their use if the mold is to be heated.