Epoxy fillets with and without epoxied glass cloth

[spirit]

I would welcome opinions about whether epoxied glass cloth is generally considered necessary to add strength to epoxy-fillets (made with epoxy resin + WEST 405 filleting blend).
My specific question concerns bonding the inside corner of right-angled joints between two pieces of 3/4" marine plywood, where the sheets are already well braced by other joints.
The outside corner of my joints will be covered with glass in epoxy.

Wizbang has said repeatedly that he uses large fillets without additional epoxied glass cloth. From the images of his boats this approach appears to work well even when the
plywood sheets are thin and stressed by substantial bending.

[Powerwagon]

The cloth spreads the load of the joint out and lowers the strain on the actual contact area. If it’s pretty strong already (i.e. braced and structurally supported) it would probably be fine.

[Benny]

My experience is very limited but I have been thinking if I were building my dinghy again I wouldn't bother....unless the joint was over say 120 degrees or something as a rule. But I suppose it depends on many variables

[cracked lid]

The point of using glass it to reduce the mass that must be added to achieve a specific joint strength. You can use a large epoxy only fillet or a smaller fillet with glass cloth. The larger the joint angle, the more weight you save using a taped fillet. If you aren't worried about weight, you can save time and effort by just using a beefier fillet.

[sp_clark]

You can use a large epoxy only fillet or a smaller fillet with glass cloth.

Pretty much my take on this also. 'Glass cloth reinforces the side of a fillet that's prone to cracking if the fillet's borderline thin. A thicker fillet won't be quite as prone to strain damage given that it usually has a wider bonded surface on the panels it spans as well as a more substantial cross-section.

Filleting panels with an angle of 120° or greater between 'em makes building a thick-enough fillet with epoxy alone a heavy, wasteful proposition, why I'd add 'glass to those. In this instance the fillet's mostly a filling, the panels and 'glass are the structural members.

[Mo 'Poxy]

I experimented fillets with/without fglass.
https://boatbw.blogspot.com/2016/02/...ze-matter.html

[neil.henderson]

If you are filleting to make a strong joint (rather than filleting to fill a joint/gap) then I would be tempted to use Wests adhesive powder rather than their filleting powder. I think you will find that the adhesive powder creates a stronger piece of cured epoxy, than if one had used the filleting powder. I only use their filleting powder where I am not concerned about strength, but where I do want something that is easy to sand after curing. For me, I do not use fibreglass mat or tape.

Regards Neil

[wizbang 13]

WEST 403 micro fibers

[spirit]

What are the "micro fibers" in WEST 403?
None of the ads that I have seen give this information...

[sp_clark]

https://www.westsystem.com/403-microfibers/

Sounds like a higher-tech version of the traditional wood flour. Cellulose fibers may not be as tough as the micro-fiber blend but can be used to thicken epoxy in a similar fashion. The Big Plus is that wet-out characteristic; you don't want bare wood pulling resin out of your thickened mixture. Why some advocate a coat of plain resin+hardener first, then fillet once that primer layer's clicked.

[David W Pratt]

#4 pretty much covers it. Gloucester Gull, IIRC, has butt joints in the plywood with a layer of glass for strength, and works fine.

[gypsie]

Where's the right angle join!?

Centre board case?
(I wouldn't glass it).

[pvg]

What are the "micro fibers" in WEST 403?
None of the ads that I have seen give this information...

I think West says somewhere that it's cotton...

[Todd Bradshaw]

403 = 65%-85% Cellulose pulp (natural substance which could come from wood, cotton, etc.) and 10%-30% amorphous pyrogenic silica. The exact WEST System mix is confidential.

https://ecoworldonline.com/what-is-c...-is-circulose/