Epoxy filler for fillets and bonds, only micro glass beads or also chopped glass fiber?

[Kris]

So it looks like there hasn't really been any discussion on what type of filler for epoxy fillets is best.

Looking at this video it seems like this "Microsphere Blend" they use is what they call micro glass beads filler here. Maybe some places also call it "Microballoons"?

The people I talked to say that this is a good filler for non-structural stuff as it makes nice smooth paste and the resulting fillet is easy to sand.

On the other hand they say to make strong fillets, it is better to add some chopped glass fiber filler, especially for structural fillets. But the end result doesn't look nice and is hard to sand.

And there is also "Colloidal Silica"? But I can't seem to find that here anywhere.

Any experience with what filler (mix) works best for fillets?

Also if I want to bond two pieces of wood together, what filler is best?

[Andy Nemier]

Great question.  Watching.  Have yet to read the Gougeon book...

[Kris]

The Gougeon book doesn't seem to mention the 409B used in the video, but it seems to confirm that "microbaloon" fillers are not the best for strong fillets.

It's a bit annoying that the book only promotes the West Systems specific brand products that are named in ways that make it hard to figure out what their composition is.

The book mentions a "microfibre" filler being best for wood bonding. I assume this could refer to pure chopped glass fibre.

For filleting specifically it refers to three different mixtures that from the description sound like varying blends of colloidal silica and what they referred to as microfibres (and possibly wood dust for colouring).

It's a bit strange that I can't find a source of this "silica" filler here if it is such a common and important epoxy filler though.

[Kris]

OK, looks like "Fumed silica" is the same (edit: or not) and available from some online retailers here in Europe. Interestingly this overview in German says that silica filler is best for bonding wood, while chopped glassfibre is not (it is however recommended for bonding FRP): https://nauticare.de/news-service/gfk-boot/fuellstoffe-fuer-epoxidh...

[Kris]

https://christinedemerchant.com/filler-silica.html

https://christinedemerchant.com/filler-micro-balloons.html

Seems to have some good tips.

[Kris]

Cabosil M5 seems to be the brand name for fumed silica that you can find online retailers in Europe for.

[Robert]

Here’s a quick reference for generic substitutes for the West System fillers:

403 For laminating/bonding porous surfaces. Milled cotton fiber or cotton flock. Maybe blend in some silica to reduce drips/sags. Great for plywood scarfs or bonding pieces of wood.

404 calcium (meta)silicate. Generic is hard to find so I just buy the West product. Indispensable for hardware bonding.

405 filleting blend. Ground walnut shells, milled cotton, fumed silica. This product is mainly about appearance for clearcoating applications. Any wood dust or powder can be substituted for walnut to color match. If painting, just use fumed silica. Blend with phenolic micro balloons to make sanding easier if you need to.

406 thickening agent/fillets. fumed/colloidal silica (Cabosil). Easy and cheaper to buy in bulk. Absorbs atmospheric moisture, so store in a tightly sealed container.

407 fairing and low density fillets. Phenolic microballoons with fumed silica added to reduce sagging. Easy and cheaper to buy phenolic in bulk. Blend in silica as needed.

409 glass bubbles (I never use this. Phenolic microballoons are better.)

410. Don’t know. I never use it. It’s too soft!

All of these fillers lend themselves well to blending to achieve intermediate properties. Especially, add silica to reduce drips and sags. Add micro balloons to make sanding easier.

Also:

Graphite powder— for making low friction wear pads such as where the beam locating blocks rub against the hull.

Chopped glass strands. Add to fumed silica to make a very strong fillet.

Note what the Gougeon brothers say about fillet density. With low density fillers such as 407 you need a thicker fillet to match the strength of a thinner, higher density fillet (403).

The best way to make a very strong and uniform fillet is to spoon thickened epoxy into a ziplock bag. Cut off a corner and use the bag to pipe the epoxy into the joint—just as the Gougeon Brothers describe with a cake decorating piping bag. Now take a length of glass tape (or, for complex curves, bias cut glass strips) and wet out with neat epoxy. Roll up the saturated strip and then unroll over the fillet. Smooth out with a gloved finger and/or an epoxy brush. This method almost completely removes the need to do any tooling. The glass turns the filled onto a very strong sandwich construction. 12oz or 17oz biax tape over a fillet makes a super strong structure.

Always wet out porous surfaces with multiple coats of neat epoxy before applying thickened epoxy or glass. Work out of direct sun in falling temperatures to avoid offgassing.

[Kris]

Excellent, that helps a lot!

[Andy Nemier]

Wow, great site.  I could get lost for days over there.     ;  )

Cheers.

Kris said:

https://christinedemerchant.com/filler-silica.html

https://christinedemerchant.com/filler-micro-balloons.html

Seems to have some good tips.

[Robert]

I should have added to the above that fumed silica is probably the best thickener for general bonding. A blend of cotton and silica is also great for wood.

[Jeff]

Off Center Harbor has a great video series for all things epoxy with Russel Brown. 

Mastering Epoxy with Russell Brown, Part 3 – Fillets, Hardeners &am...

In this video he says he does a blend of 404, 406 & 407 for his fillets.

1 part 404, 2 parts 406 and 2 parts 407.

Any thoughts from the experienced builders here?

[Kris]

I ended up ordering some bulk Cabosil M5 and also a small quantity of West Systems 403 and 409 to test. Should arrive in a few days.

The package of the 409 says it is a compromise between relative strength and easy workability by hand, so that is probably why it is used in that official Wharram video.

I could have also ordered phenolic microballoons, but they are easily double the price of the glass beads.

[Kris]

This video shows how rough fillets come out with silica and chopped glass fiber, but apparently you can smooth it over with a glass fabric quite well: https://youtu.be/9jHUI0CiYbs