Thickening epoxy with fillers and additives

Select a filler for the job, then thicken the mixture to the consistency the job needs. The amount of filler controls the thickness — there is no strict formula, and your eye is the instrument.

1. Mix resin and hardener first

   Mix the quantity of resin and hardener you need thoroughly, before any filler goes in. Start with a small batch and leave room in the pot for the filler.

2. Blend in the filler

   Add the filler in small handfuls or scoops until the mixture reaches the consistency you want. Make sure all of it is thoroughly blended before you apply the mixture.

For maximum strength, add only enough filler to completely bridge the gap between the surfaces without sagging or running out of the joint. A small amount should squeeze out when the joint is clamped. For thick mixtures, do not fill the pot more than a third full of epoxy before adding filler.

For fairing compounds, stir in as much 417 Low Density Filler or 410 Microlight Filler as will blend in smoothly — the thicker the mix, the easier the sanding. Spread the mixture into a thinner layer, around the inside of the pot or onto a flat non-porous palette, to extend its working life.

WEST SYSTEM instructional material written in the United States refers to US filler numbers. The range ATL Composites supplies in Australia uses different numbers for the same jobs.

Equivalents from the ATL WEST SYSTEM 400 Fillers page. Confirm the selection for your application against the current technical data sheet, or ask ATL's technical team.

A drill-motor mixer at high speed throws lightweight filler into the air and flings clumps of thickened epoxy around the shop. Planning the set-up is most of the job.

Setting up

• Choose the location — out of the way, but close enough for easy access to the project, the epoxy and the fillers.
• Mask the floor with cardboard or a plastic drop sheet, and tape the edges down so nobody trips.
• Use a 19–26.5 L lidded plastic pail. Cut an opening in the centre of the lid just large enough for the mixer shaft — a narrow slot if the mixer is a flat, paint-roller type. The lid keeps lightweight filler in the bucket, so keep the opening small.
• Use a paddle 300 mm longer than the container is tall.
• Use a powerful drill motor. Cheap drills burn out after a few batches of thickened epoxy.
• Protect the drill with wax and duct tape, and stretch a latex glove over the handle and trigger — epoxy drool locks triggers up. Leave the cooling vents clear.

Work to a recipe

If the project needs several batches of the same material, settle on a recipe: so much epoxy, so much filler per batch. Add measured amounts to the first batch until the viscosity suits the job, keeping track of the total. Every later batch can then take all its filler at once and come out the same.

The WEST SYSTEM Technical Manual & Product Guide includes a Filler Proportion Guide with starting quantities for each filler and viscosity. Treat it as a guide, and still thicken the first batch in stages.

Mixing the batch

1. Mix the resin and hardener thoroughly, scraping the corners and sides with the paddle.
2. Add the measured amount of filler to the bucket.
3. Cover the bucket with the slotted lid.
4. Insert the mixer through the lid and into the epoxy.
5. Start slowly, then gradually increase speed.
6. Move the mixer up and down and side to side. The motor will labour at first and ease as the mix turns smooth and creamy.
7. Remove the lid and mixer.
8. Scrape the corners of the bucket so no unthickened epoxy is left behind.
9. Return the mixer and stir briefly until uniform.

Quality control matters more than equipment: the resin and hardener must be mixed thoroughly, and a powerful mixer will not compensate for a poor ratio or a rushed mix. Method adapted from Big Batch Mixing Methods by Tom Pawlak, Epoxyworks issue 9.