First of all make certain the mould is clean by washing with soap and warm water and then dry thoroughly. The mould may be any found object and can be made of metal, plaster or plaster.
Applying release agents
Two coats of a release agent such as paste wax should now be applied to the mould, following the directions on the can. Allow the wax to dry for a least 30 minutes.
Next apply a PVA release coat, using a soft sponge as brushes tend to leave streaks that show up in the finished product. Make certain that you cover the entire surface of the mould evenly and do not leave any runs as these will also show up in the article you are making. Allow to dry, if necessary use a hair dryer to speed the process up.
Applying the gelcoat.
Step 1. Estimate the amount of gelcoat required for the job.
On average 500ml of gelcoat are required for 1 square metre of mould surface. For half a square metre use 250ml of gelcoat.
Pour the gelcoat into a mixing container. Use a waxed paper drinking cup or something similar for small amounts and an ice cream container is ideal for larger jobs.
Step 2. Estimate the amount of catalyst required.
The MEKP catalyst used is dangerous and small amounts should be measured using a plastic pipette or dropper bottle. Use 1.5% catalyst on a hot day and 2% on a cold day. For amounts over 3ml use a small conical measure.
For 1litre gelcoat on a cold day you would require 20ml catalyst at 2% .
For 500ml gelcoat you would require 10ml of catalyst.
Step 3. Pour the catalyst into the resin and mix using a flat stick.
Applying the gelcoat to the mould
Apply a thick coat of the gelcoat to the mould using a clean brush in quick even strokes as the gelcoat will start to set in 10 minutes. The coat should be about 0.5mm thick. Even the coat out by going over thin and thick areas until you are satisfied that it is as even as you can make it.
Clean the brush by washing in acetone. Allow the gelcoat to become almost touch dry, before starting to apply the glass. This will take between 2 to 6 hours depending on the temperature.
Preparing the chopped strand mat
For most small jobs 2 layers of 225gm chopped strand mat will be sufficient. The most commonly used mats are from 225gm to 600gm of weight per square metre. When getting the mat ready by cutting and tearing allow an extra 10-15mm around the edges. Stack the mat on some clean newspaper besides the job.
Estimating the quantity of resin required.
On average you will need 500ml of resin for each square metre of 225gm chopped strand mat. If using a heavier mat for your project proportionately increase the amount of resin. For instance for 1 square metre of 450gm mat you would need 1litre of resin.
Applying the mat
Step 1. Pour the MEKP catalyst into the resin and mix using a flat wooden stick, making certain to scrape the sides and bottom of the mixing container while stirring to ensure proper mixing.
Step 2. Now brush a thick coat of the resin on top of the gelcoat in the mould.
Step 3. Place the glass in the mould on top of the wet resin you have just applied and move into position to cover the entire mould surface.
Step 4. Apply another coat of resin. Rather than use a brushing action which could move the mat out of position dab the resin onto the mat. Make certain that you wet the entire mat surface with resin and don’t leave any dry areas.
Step 5. Wait a few minutes for the resin to penetrate the mat thoroughly and then consolidate the lamination you are making, by gently rolling over the surface with a metal roller. This will help to remove air bubbles and thus improve the quality of the laminate. Keep the roller clean to avoid pick up of the mat by rinsing in acetone.
Step 6. Add the 2nd and any additional layers of mat required in the same manner and allow the job to cure overnight. Clean your brush and roller in acetone.
Removing the casting
Remove the casting from the mould by using a thin wooden wedge and slip it under one side of the job and work it around the edge, until the project springs free of the mould.
Trim all the edges using a hacksaw held at a 45 degree angle. This will give a thin edge that can be filed and then sanded using 80 grit sandpaper.
Wash the casting under water to remove the water miscible PVA release agent that will still be on the surface.
If desired you can then coat the surface with release agents and repeat the process described above to yield an exact duplicate of the mould used.
If a smooth finish is required for the inner surface of the finished project, apply a finishing coat of flowcoat resin to the laminate you have just completed. You will need half the amount of flowcoat as previously measured for gelcoat and 1.5% MEKP catalyst on a hot day and 2% on a cold day. Mix and brush on in long but even strokes of the brush without wasting time, as it will take only about 10 to 15 minutes for the flowcoat to gel. Set aside to dry and clean your brush in acetone.
Do not guess the quantities of resin or catalyst required for each step of the process use accurate measures and you will have no problems making many useful and attractive fiberglass items.
A full list of items needed in this article can be found on our webstore at www.aldaxstore.com.au
Stan Alderson is a chemist and researcher with over 25 years practical mouldmaking and casting experience. He has a particular interest in assisting new business entrepreneurs. Keep in touch with new developments at Stan’s “Mould Making and Casting Blog”