West System 404 High-Density Adhesive Filler

SKU:SWES-404-15

$31.75 USD

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Size: 15.2 oz (404-15)

15.2 oz (404-15) 43 oz (404-45) 30 lb (404-B)

Quantity

404 is the highest strength adhesive filler in the West System range, built for joints that carry real load rather than joints you plan to sand. Engine mounts, chainplates, drilled-and-filled fastener holes, hardware bedding. Anywhere the cured joint sees compression and you want the epoxy to behave like structure rather than glue.

Thicken mixed 105 epoxy with 404 until it holds a peak, then fill the bore or bed the hardware and let it reach working strength before loading.
The drill-oversize, fill, redrill treatment on deck hardware is exactly what this filler is for: you end up fastening into cured epoxy rather than into core.
Not a fairing filler. It cures hard and sanding it is genuinely unpleasant, which is the tradeoff for the strength.
Heavy density class, off-white in color, formulated for epoxy.
Sizes: 15.2 oz (404-15) and 43 oz (404-45) cans and a 30 lb box (404-B).

Mix it stiffer than you think you need for a vertical bedding job.

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Technical Specifications

Made in USAWorks with Epoxy

Filler TypeHigh-density structural adhesive filler

Physical FormDry powder

ColorOff-white

Density ClassHeavy

SandabilityNot for fairing

Compatible ResinsEpoxy

Country of OriginUSA

Net Weight by Size

404-1515.2 oz (430 g) can

404-4543 oz (1219 g) can

404-B30 lb (13.6 kg) box

Compare Fillers

Product	West System 404 High-Density Adhesive FillerYou're viewingfrom $31.75	1/32" Milled Fiberfrom $14.95	Aerosil - Cabosil (Fumed Silica)from $9.95
Best for	High-density structural adhesive filler	Thickening / reinforcing filler	Thixotropic thickener
Density	Heavy	Medium	Light
Sandability	Not for fairing	Moderate	Not for fairing
Resins	Epoxy	Epoxy, Polyester, Vinyl Ester	Epoxy, Polyester, Vinyl Ester
Material	N/A	Milled E-glass fiber (1/32 in)	Amorphous fumed silica (SiO2 >99.8%)
	Selected	View	View

Which epoxy filler do I use for fairing, bonding, or filleting?

Match the filler to the job, and keep strength fillers below the surface with fairing fillers on top:

Bonding and structural fillets: use a high-density or fiber filler like colloidal silica (Cabosil), milled fiber, or microfibers, which cure hard and strong.
Fairing and shaping: use a low-density filler like phenolic microballoons, glass bubbles, or a fairing compound, which sand and feather easily.

What's the difference between a fairing filler and an adhesive filler?

Adhesive, high-density fillers like colloidal silica and microfibers make a hard, strong mix for bonding and structural filleting, but they are tough to sand. Fairing, low-density fillers like microballoons and glass bubbles make a light, soft mix that sands and feathers easily for a smooth surface, but they are not structural. Use adhesive fillers for strength and fairing fillers for finish.

How much filler do I add to epoxy or polyester resin?

There is no fixed ratio. Mix the resin and hardener fully first, then stir in filler a little at a time until you reach the consistency the job needs:

Ketchup consistency: for a coating or pre-coat that still flows.
Mayonnaise consistency: for bonding parts together.
Peanut-butter consistency that holds a peak: for filleting and gap-filling.

Which filler makes the easiest-to-sand fairing compound?

Low-density microsphere fillers sand the easiest. Phenolic microballoons, 3M glass bubbles, and lightweight blends like our fairing compound or West System 410 Microlight feather to a fine edge and carve with little effort once cured. They trade strength for sandability, so use them above the structural layer for shaping and smoothing, not for bonding or load-bearing fillets.

Are epoxy fillers structural?

Some are. High-density and fiber fillers like colloidal silica, milled glass fiber, and microfibers add real strength and are made for bonding and load-bearing fillets. Low-density fairing fillers like microballoons and glass bubbles are not structural; they are for shaping and fairing only. Build strength with a high-density filler first, then fair over it with a low-density filler.

What's the difference between West System 403, 404, 405, 406, 407, and 410?

Each West System filler is tuned to a different job:

406 Colloidal Silica: the versatile thickener for bonding and filleting; controls sag on vertical and overhead work.
404 High Density: the strongest, for hardware bonding under high load.
403 Microfibers: add toughness for structural filleting and gap-filling.
405 Filleting Blend: a natural wood-tone fillet that sands easily.
407 Low-Density: the microballoon fairing filler that is stronger than 410 and still easy to sand and carve; cures dark red-brown.
410 Microlight: the low-density fairing filler that sands the easiest over large areas.