Epoxy Fillets

[frasertheking]

I have seen many people build aquariums using epoxy but not often do I see people using epoxy fillets. They have many advantages but mainly they add a large amount of strength when doing construction with plywood. I find most of my information from a boat building side of epoxy use but it would be helpful here I think. The basic use goes:

1. mix up your epoxy and hardener fully, I aim for 2 minutes. Save a small amount or if you are quick lightly wet out the joint with unthickend mix.

2. Add you thickener, You can use wood flour/sawdust but for the small work people do buying a premade thickener like offered by west coast is not too pricey and they have many choices.
Micro balloons make a thick paste that is easy to sand
Microfibers add a decent amount of strength
Colloidal Silica makes a very tough but hard to sand fillet.

3. If you have not done so lightly wet out both sides of the joint with unmodified epoxy, this lets the surface absorb some of the epoxy and prevents the joint drying out.

4. use a lollypop stick or just a gloved finger to form a rounded fillet in the joint, just pack it in. Try not to go over 1/2 inch thick as the heat generated can get out of hand.

As I said this is mainly used in boat construction but you can modify pretty much any epoxy from a light slurry to the thickness of peanut butter, the thicker you go the more strength you lose so bare that in mind. I will post some good resources I found at the bottom and I am not a expert by any means so if this is common knowledge skip over but I can answer simple questions . West Coast has great guides but you pay a premium for it. This applies to almost all epoxies.

Epoxy Fillet

4. How to make the perfect epoxy fillet - Practical Boat Owner

David Pugh describes the process required to produce accurate coves while keeping mess and waste to a minimum

www.pbo.co.uk

Thickening epoxy

Home | WEST SYSTEM Epoxy

The home of WEST SYSTEM marine-grade epoxy resins and hardeners for boat building, boat repair, home repair, and restoration.

www.westsystem.com

A great free book on boat building but most techniques carry over

Gougeon Brothers On Boat Construction Book | WEST SYSTEM Epoxy

Download a free PDF of the Gougeon Brothers On Boat Construction book in its entirety, or order it from Amazon.com.

www.westsystem.com

Some nice intro videos on the use of epoxy

 

[Dovii kid]

Good info here. I’ve done a lot of research in preparation and will be using fillets prior to laying my Biaxial cloth in the seams

 

[hartebreak]

Do you then glass over the fillet or is the fillet enough?

 

[the_deeb]

I usually glass over the fillet. I find the larger radius of the fillet makes it easier to get a clean bubble-free layer than if you’re trying to squeeze your fiberglass cloth directly into the sharp corner. Another advantage of the fillets.

 

[frasertheking]

Do you then glass over the fillet or is the fillet enough?

You can glass over the fillet or use fiberglass tape on the corners similar to stitch and glue construction that should be outlined in the free book. The fillet adds a huge amount of strength in general due to the geometry. it can get a bit confusing but when you glue a joint it is actually much stronger than fastners but it is only as strong as the maximum strength at the point under load as i understand it much like a screw is only as strong as the material it is biting into. it spreads the load out along the entire fillet and can near double the strength of a joint.

it is important you get the right tape/fibers for epoxy too because glass used with polyester is coated in styrene while this is not present in glass made for epoxy.

 

[hartebreak]

Next question about doing a fillet. I am wanting to coat my tank with Macropoxy 646. Can I use that to make the fillet "peanut butter" ? If not, would there be an adhesion issue if I used 1 type of epoxy for the fillet and Maceopoxy for the coating?

 

[frasertheking]

Next question about doing a fillet. I am wanting to coat my tank with Macropoxy 646. Can I use that to make the fillet "peanut butter" ? If not, would there be an adhesion issue if I used 1 type of epoxy for the fillet and Maceopoxy for the coating?

I think that would be one for the manufacturer or a rep for the company if you do not plan to use normal epoxy first. it might work but if it is formulated to be painted on it might do something odd. I mixed up a batch of thickened epoxy and messed up the mix and the tub I was using and started smoking and nearly caught fire, started boiling and set in about 2 minutes. So it is always best to check haha.