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

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?

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Permalink Reply by Kris on October 19, 2024 at 11:02am

So, after using the stuff a bit:

the 409 works great as a filler for repairs, but to prevent it from dripping on non horizontal area you need to add a lot which results it in being quite weak in the end. Good for non-structural things, but not exactly cheap.

The Cabosil-m5 works fine as a thickener, but it is a bit annoying that it seems to introduce small air bubbles even without mixing it very strongly, and those small bubbles only partially leave before the epoxy hardens.

The 403 micro-fibers seems to be just cotton fibers or so and I don't see much advantage of them over the cabosil to be honest. Maybe they make for slightly stronger joints?