Additive of the Month – 421 Fire Retardant Additive


What is it? WEST SYSTEM® 421 Fire Retardant Additive does what it says on the tin – it retards fire. It is a heavy, non-combustible powder that is added to a fresh mix of epoxy.

Used for? An extra layer of fire protection in areas such as galleys and engine rooms, or anywhere there may be a risk of heat absorption that could lead to combustion, including cabins and saloons. It could also be used to line the coffer dams around skin fittings to prevent hoses being burnt through, or to run on partitions beside hot areas such as exhausts. It is especially useful if the partitions themselves are combustible, such as plywood bulkheads or galley headliners.

Mixing ratio: An easy 1:1 by weight. However, at this ratio the additive will greatly increase the viscosity of the epoxy so the resulting mix will need to be applied with a trowel or squeegee.

 WEST SYSTEM® 421 Fire Retardant Additive before being added to epoxy resin

The density of the retardant means it requires very thorough mixing into the resin

Hamish’s Tips

“This is a very straightforward additive, so just follow the mixing ratio and be prepared to apply a thick layer of material,” Hamish says.

“It will help to apply a coat of epoxy onto a properly prepared (degreased and abraded) substrate first to achieve better adhesion. When tacky, apply the epoxy/421 Fire Retardant Additive mix over the top. If you want to apply further layers of sound-proofing to the partition use peel ply during the application or wash the cured 421 Fire Retardant Additive down to remove the amine blush before sanding.

“Key the surface to promote good adhesion of the soundproofing, although peel ply does that automatically on removal. That’s it – 421 Fire Retardant Additive will give additional peace of mind in areas of the hull with a higher risk of fire.”

Typical application:

We wanted to enclose the engine wet exhaust that runs along the floor between two bunks, so needed a plywood partition. To fireproof the plywood beneath the soundproofing, we decided to treat it with a coat of 421 Fire Retardant Additive.

Fireproof plywood using  WEST SYSTEM® 421 Fire Retardant Additive and epoxy resin

The first job was to roll on a coat of WEST SYSTEM 105 Epoxy Resin with 205 Fast Hardener. This would act as a primer layer and seal the wood.

how to prime wood with epoxy resin and roller

Whilst still tacky, we then added a layer of glass fibre reinforcement cloth, partly to strengthen the plywood but also to act as a mat for the 421 Fire Retardant Additive.

how to add a layer of glass fibre reinforcement cloth to act as a mat for the 421 Fire Retardant Additive.

We then mixed up the fire retardant with mixed epoxy to achieve a spreadable mix. It is possible to alter the ratio to change the consistency, so it can be made thicker for easier trowelling (such as if applying overhead) or thinner for easier brush application, but the manufacturers recommend the ideal is 1:1 by weight which will give a spreadable paste.

Adding the mixed up fire retardant with epoxy resin

The partition with one relatively thin layer of 421 Fire Retardant Additive applied by squeegee. Once cured, more retardant mix can be added if either peel ply was used, or if the amine blush is washed off and the surface abraded.


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