Additive of the Month – 420 Aluminium Powder


What is it? WEST SYSTEM 420 Aluminium Powder is made from high quality aluminium that has been very finely ground. It is available in a range of tub sizes.

Used for? Additives can have significant effects on basic epoxy and with 420 Aluminium Powder the metal acts as an effective UV barrier. It also gives the coating a silver metallic finish and makes it significantly harder. As such, 420 Aluminium Powder is usually used where a part will be exposed to indirect sunlight and because of the hardness it brings, 420 Aluminium Powder is sometimes used as a decorative tip to a rudder or foil. The powder also makes an attractive finish to moulded items such as engine trays.

Mixing ratio: Add between 5% and 10% by volume to the epoxy/hardener mix.

Hamish’s tips:

Wear personal protection equipment:

“Aluminium is generally harmless when kept outside the body but this is a very fine powder so you don’t want to breathe it in. Make sure it doesn’t get into any foodstuffs or drinks either. Wear disposable gloves and keep it away from your coffee cup.”

Mix with a dabbing motion

“As with all fine powders, the best way to mix it in with epoxy resin is by a dabbing motion to start with. Stirring rapidly can cause the material to fly up into the air. You can stir once the dabbing has caused most of the powder to mix into the resin.”

Use in last layer as pigment

“If using the aluminium as a pigment for a metallic finish, use it in the final layer, as the thickened nature will prevent it from sinking fully into the substrate. If you are coating plywood, for example, you want good resin saturation, so hold the aluminium back to the final layers.”

It’s good – but not for everything

420 Aluminium Powder has some great features, mainly in UV protection and for impact resistance, but other additives may be better suited to specific tasks. On rudders and centreplates, for example, 423 Graphite Powder will also offer impact resistance whilst providing additional low friction resistance to improve boat speed. Similarly, when coating items such as battery boxes, 421 Fire Retardant additive may prove better, although 420 Aluminium Powder will still do an effective job. That is the beauty of epoxy – it’s a very versatile material and can absorb a wide range of other materials without an adverse reaction.”

(If you’re in need of extra advice, please ring Wessex Resins and Adhesives technical helpline on + 44 (0) 1794 521111)

Using 420 Aluminium Powder:

Coat the surface

The surface to be coated should be lightly abraded to remove any gloss and to provide a key. This piece of GRP is being sanded with 180 grit, a process that would be used if it was the tip of a rudder or centreplate.

Mix epoxies together

WEST SYSTEM 105 Epoxy Resin and 205 Fast Hardener are mixed together at the correct ratio of 5:1, as seen here. (25ml of resin has received 5ml of hardener to make 30ml in total).

Add aluminium powder

The aluminium powder is added to the pre-mixed epoxy and introduced with a dabbing motion until absorbed to prevent any flyaway. It is then fully stirred in, with the corner of the stick scraped into the bottom edge of the pot for a complete mix.

Apply first coat

The first coat can be applied to the GRP in the usual way by using a roller.

Smooth the coat

To smooth the coat a foam sponge brush can be lightly run over the surface.

Apply further coats

A second or even a third coat can then be applied ‘wet-on-tacky’ for a richer finish.

See the result

The result should be a homogenous mix with a distinctive metallic finish. You can apply a few layers of UV resistant clear varnish over the top if you want the piece to be decorative.

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