Epoxy fillets ingridients confusion

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Mario76
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Epoxy fillets ingridients confusion

Post by Mario76 »

To make fillets are used (apart of epoxy) different stuff: wood flour, silica , microspere (micro bubbles).
Which one is best for strong fillets?

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Re: Epoxy fillets ingridients confusion

Post by gonandkarl »

Hi,
I used all three of them for all my boats fillets always 40 % woodflour 40 % microballoons and 20 % silica and mostly I achieved that by adding 1 teaspoon full of silica and 2 for wood flour and micro balloons. I stirred the ingredients into the epoxy after the epoxy was stirred for 5 minutes by itself. I repeated the teaspoon procedure until the fillet mixture was a nice paste not too thick and not to thin. I cannot guarantee that this is the correct mixture but maybe some other boatbuilders will chip in and give you a better recipe.
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Re: Epoxy fillets ingridients confusion

Post by fallguy1000 »

A fillet made with balloons is highly sandable; this is because the volume of epoxy is rather low in the mixture. And this means the strength of a balloon mix is not as strong.

What is important to understand and relate in the issue is the purpose of the fillet.

Most of the time, the fillet serves no structural purpose and you could make them from epoxy mixed with jello and achieve the need which is to make the glass bond all the way across the joint in tabbing and to keep the strength fairly consistent.

But.

The material used by the builder must also satisfy the builder's needs. For me, I found that microballoons, for tabbing, drove me up a wall. I could never keep them from sagging and they would pull and behave oddly. So I went to fumed silica 100%.

But.

Fumed silica is an incredible bitch to sand.

So.

I started to tab immediately over these silica mixes, no sanding.

I also did some testing of various mixes in the shape of a fillet and I found that straight silica was really strong. But you can also test a few different options. I don't have wood flour here.

Having the mixes sag too much or fall from the joints means you did not use enough silica. If you want a plywood edge to bond well, best to precoat and cure to green stage before bonding as well.

Now, for fairing mixes...

I use Silvertip epoxy. 548g mixed resin, 383,165 25g fumed silica, 75g balloons. This mix may sag in high heat or of you use a lot of it over and over. Adjust accordingly...but you are not ready for fairing..
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Re: Epoxy fillets ingridients confusion

Post by TomW1 »

Mario76, Tom here, filets need different components. If your joining the panels on the bottom of the boat with 12oz 6" wide biaxial tape then you only need epoxy as it will covered by another layer of 12oz biax cloth or more depending on the boat. For the edge filets and some other areas again using 12oz biax you will use epoxy with a mix of silica and micro balloons. The silica provides the strength and non-sagging property while the balloons allow it to be sanded. The store here sells it pre-mixed. Fallguys mix sounds pretty good also. If you have a lot of heat up the silica a little, but not to much. He is in Minnesota after all. :D

Hope that helps a little, Tom
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Re: Epoxy fillets ingridients confusion

Post by joe2700 »

fallguy1000 wrote: Sun Mar 28, 2021 9:45 am
What is important to understand and relate in the issue is the purpose of the fillet.

Most of the time, the fillet serves no structural purpose and you could make them from epoxy mixed with jello and achieve the need which is to make the glass bond all the way across the joint in tabbing and to keep the strength fairly consistent.
I'm pretty certain an epoxy fillet of appropriate material actually adds significant strength compared to an epoxy glued joint with no fillet. It increases the surface area of the joint by quite a bit. It may be true that the designer is not counting on that strength, only relying on the fiberglass that goes over the top. But since you have the fillet anyway you might as well use something dense and tough enough to add strength, ie a structural filler not a fairing filler.

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Re: Epoxy fillets ingridients confusion

Post by fallguy1000 »

joe2700 wrote: Mon Mar 29, 2021 9:51 am
fallguy1000 wrote: Sun Mar 28, 2021 9:45 am
What is important to understand and relate in the issue is the purpose of the fillet.

Most of the time, the fillet serves no structural purpose and you could make them from epoxy mixed with jello and achieve the need which is to make the glass bond all the way across the joint in tabbing and to keep the strength fairly consistent.
I'm pretty certain an epoxy fillet of appropriate material actually adds significant strength compared to an epoxy glued joint with no fillet. It increases the surface area of the joint by quite a bit. It may be true that the designer is not counting on that strength, only relying on the fiberglass that goes over the top. But since you have the fillet anyway you might as well use something dense and tough enough to add strength, ie a structural filler not a fairing filler.
The primary purpose is to allow for a void free transistion between planes because glass does not bend into and thus bond in sharp corners. The surface area of a typical joint is actually reduced by fillets. If the length of the fillet is 1/4 of the circumference of a circle of radius r or pi*r/2, the length of the same space without a fillet is 2r (more). The opposite is true for outside corners. A fillet is less surface area than none.

The reason for fillers in this bond space is because epoxy will exotherm and heat a lot and is too viscous. The fillers can be tested for various engineering attributes, but the epoxy is the primary bonding agent. You could use sand itself, but messy and heavy, etc. You could use jello/sugar, but also heavy. The bond strength of those fillers is largely irrelevant and this is why we see so many options given for fillet materials.

I wish I was wrong!

I did plenty of research and found very little information pointing me to which way was best. I did find that fumed silica made silky smooth fillets and did not clump or make lumps. Cotton was clumpy and is hygroscopic. Corecell grindings were light, but remained pliable in the joint. Microballoons sagged too easily. I did not test wood flour. Epoxy and milled glass was very heavy, but hard like a nail and would be resistant to bottom or dock impacts.

If you can find any data on this, let me know. I saw a few commercial sites touting the use of balloons, but after hundreds of feets of fillets; I learned this was not good and sagged too much.
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Re: Epoxy fillets ingridients confusion

Post by BrianC »

There are a few designs like the SK-14 where the fillets are indeed structural -- no tape is used. I'd keep the balloons out of the mix for those applications.
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Re: Epoxy fillets ingridients confusion

Post by joe2700 »

fallguy1000 wrote: Mon Mar 29, 2021 10:11 am
The primary purpose is to allow for a void free transistion between planes because glass does not bend into and thus bond in sharp corners. The surface area of a typical joint is actually reduced by fillets. If the length of the fillet is 1/4 of the circumference of a circle of radius r or pi*r/2, the length of the same space without a fillet is 2r (more). The opposite is true for outside corners. A fillet is less surface area than none.

The reason for fillers in this bond space is because epoxy will exotherm and heat a lot and is too viscous. The fillers can be tested for various engineering attributes, but the epoxy is the primary bonding agent. You could use sand itself, but messy and heavy, etc. You could use jello/sugar, but also heavy. The bond strength of those fillers is largely irrelevant and this is why we see so many options given for fillet materials.

I wish I was wrong!

I did plenty of research and found very little information pointing me to which way was best. I did find that fumed silica made silky smooth fillets and did not clump or make lumps. Cotton was clumpy and is hygroscopic. Corecell grindings were light, but remained pliable in the joint. Microballoons sagged too easily. I did not test wood flour. Epoxy and milled glass was very heavy, but hard like a nail and would be resistant to bottom or dock impacts.

If you can find any data on this, let me know. I saw a few commercial sites touting the use of balloons, but after hundreds of feets of fillets; I learned this was not good and sagged too much.


It seems you are talking about the surface area available for the glass tape, but since I was comparing to a joint without glass tape I was talking about the area that is glued with or without a fillet. Let me try to say what I mean more clearly.

Let's say we are joining 2 pieces of ply at a right angle with epoxy. There are 3 options:
1 - thickened epoxy between the pieces
2 - thickened epoxy between the pieces with a thickened epoxy fillet on each side
3 - thickened epoxy between the pieces with a thickened epoxy fillet on each side, and 1 layer biaxial tape each side.

Option 3 is certainly the strongest. That said option 2 is significantly stronger than option 1 as the epoxy is bonding to a much larger area of the substrate because of the fillet. A fillet adds significant strength even without glass. You are saying the bond strength of the fillet doesn't matter which I accept since epoxy mixed with most normal things is stronger than the underlying plywood anyway. However in my experience fairing fillers are softer and more brittle than structural fillers. I'm saying I think the structural propertied of the fillet material will have some impact on the joint strength, but perhaps not the adhesive properties. I think an identical joint with a wood flour fillet would be stronger than one with a microballoon fillet.

To be clear I'm stating it as a fact that the fillet alone adds significant strength to a joint, but it's my hypothesis that properties of the fillet material have an impact on that strength.

If you check out literature from epoxy manufacturers they speak about the strength of fillets alone, and the added strength of covering them with glass, indicating they intend for them to be sometimes used alone.

For what its worth wood flour or similar seems to be the standard for fillets. West 405 Filleting Blend is mostly walnut shell flour, System 3 ez fillet is wood flour.

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Re: Epoxy fillets ingridients confusion

Post by cape man »

Not to add another answer to what was a simple question... but I prefer wood flour mixed with epoxy to the consistency of thick peanut butter for my filets. No sanding required if you apply the fiberglass within 24 hours of the filet. Wood flour and epoxy IS very strong, and perhaps the cheapest filler you can find (it is the cheapest sold here). For structural components like filets and overfilling holes that will then hold a screw, wood flour is really great stuff.

It is harder to sand than some of the other fillers that are more suitable for fairing compounds.
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Re: Epoxy fillets ingridients confusion

Post by OneWayTraffic »

The Gougeon Book has a run down. Obviously they talk about their own fillers and epoxy but the principles are the same.

https://www.westsystem.com/wp-content/u ... 1205-1.pdf page 102.

In a nutshell you can use fillets of any sensible filler to produce bonds stronger than the plywood. Lower density fillers need bigger fillets to achieve equivalent strength.
For example: 6mm ply 18mm radius for a high density filler(silica), and 44mm for a low density (microballoons).

If putting glass tape over then the tape carries the load, so it doesn't matter as long as you have enough radius to carry the glass. I use a 1:1:1 mix of microballoons, microfibres and silica, then lay tape over immediately. I feel that some microballoons adds a little flex to the joint, though it doesn't really matter with two or three layers of glass running over. If I wasn't glassing it I'd use a small fillet of silica then go over with low density when green for sanding later.

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