Additive of the Month – November 2018


HIGH DENSITY FILLER (Type: Finely milled adhesive filler. Product No: 404)


What is it?  Originally designed to bond the fixings into wooden blades used to attach wind turbine hubs, 404 High Density Filler is a very fine powder used for bonding hardware into substrates. When mixed with WEST SYSTEM epoxy, the cured material is highly resistant to compression and high cyclic loading.

Used for? 404’s resistance to any form of distortion makes it ideal for filling voids that are going to be subjected to big loads. Prime examples would be padding out the gaps beneath a winch drum, or replacing a rotted balsa core deck under a cleat. It can also be used for filleting and gap filling.

Mixing ratio: The High Density Filler can be added to a WEST SYSTEM resin and hardener mix at anywhere between 35% to 60% by weight, depending on the viscosity required. A thick paste, towards the 60% end, would typically be used where access is easy and the ‘peanut butter consistency’ paste can be applied with a palette knife. The more fluid mix would be used where it has to be decanted or injected, such as into a bolt hole.

Hamish’s Tip’s for the epoxy/404 blend:

“The 404 High Density Filler is the real workhorse of hardware bonding,” Hamish says. “Once cured, it is remarkably strong and can withstand loadings of several tons. When a deck fitting is faced with excessive loads, it is usually the substrate that gives way before the cured epoxy/404 mix.”

Wax the fittings you may want to remove

“Because 404 is an adhesive filler, it will glue itself to any non-slippery surface. If you want to use it with hardware you may want to remove later, you will have to grease the contact surfaces. We recommend using a mould release wax such as Maguire’s. Coat the threads using a cloth but then remove the excess in the threads by pushing a fingernail through the cloth and into the groove. (Excess wax will make the grip less effective as it will displace the 404.)

Use the ‘x times 6mm’ rule


If you plan on capturing a fitting, then a useful guide is to drill an oversized hole 6mm wider than the bolts or screws you intend to use. This will give 3mm around the thread for the epoxy/404 mix to occupy. The photo shows a cutaway of a typical bolt hole. Note the end of the bolt sits in a pilot hole to keep the bolt upright, whilst the upper part is a void with 3mm all round. Once filled with epoxy/404, the bolt will be held rigidly in place but could be extracted later if properly greased. The epoxy/filler mix will have moulded itself as a thread around the bolt. Please note that the nut can be encapsulated too, a good way to hold the bolt vertical, or left on the outside if preferred to aid extraction.

The ‘X plus 6mm’ ratio works well for most sizes of deck fitting. If the hole is too big then the large column of epoxy mix needed may well ‘exotherm’ once it is poured into the hole. This self-generation of heat creates air bubbles and could compromise the filler.

The WEST SYSTEM manual gives a clear step-by-step as to how to use use 404 High Density Filler for a variety of jobs.

It’s tough stuff!

If you doubt that a powder mixed into epoxy can be strong, think again! These two stainless steel eyebolts are identical and were both encapsulated into a pine block with epoxy/404 High Density Filler mix. It took 1.25 tonnes of vertical pull to break the one on the left out of the block and in doing so the eye itself was severely distorted. The epoxy remained intact around the bolt – it was the wood itself that failed!

A typical application:

This mock up shows how epoxy/404 mix is regularly used to change the angle of deck fittings, useful on canted decks. The ideal position of the base plate is established using a wooden wedge and the ends built up using small peaks of epoxy thickened with 404 Filler. Once cured, the wood (which avoided being coated) is extracted and the rest of the void is filled. The result is a solid, compression-resistant platform of great tensile strength.


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