Alternative fillet materials?

[Ward]

I'm currently building a small flat-bottom 12' canoe, actually i just started on it yesterday. I'm currently in the process of scarfing the side panels together, so I won't need fillets for a little while. Most of the sites I've seen had people using either wood flour or microbaloons mixed with epoxy for fillets. I can't find either an any stores around here. I did see one guy's site though, who was building pretty much the same design as me, just a dirt cheap, simple, 3-piece flat bottom canoe. He used bondo for fillets, and it looked like it worked pretty well. He still covered the fillets with glass tape, too. Would this be okay? I have enough spare bondo laying around the garage, since my dad and I just finished the bodywork and paint on my sisters car. This canoe will never see 'rough' waters, its going to be used on a lake near my house. I doub't if I'll even take it out in whitecaps, because i don't think it would be fun fighting the wind with big flat sides. Are there any other alternatives? Cheap is preferrable too, im tryig to build this thing for around $100.

[gonzo]

Bondo is OK for fillets in a canoe. If you fiberglass the seams, the bondo will be reinforced sufficiently. I have had success with it.

[Ward]

What about not using fillets at all? Could I just use glass tape on the seams, inside and out?

[SailDesign]

Without fillets, the angle/radius on the inside corners would be too small, and the tape won't take. The fillet is mainly for this reason, not as a structural/glue element.
Steve

[guest]

I'm currently experimenting with PL premium plus - masonary crack sealant.
it's about 2.50 for a grease gun tube. Very adheasive, very light, and dries "spongy", but a little hard to work with. Runy and seems to stay sticky forever.

[JR-Shine]

Epoxy and polyester (bondo) do not bond very well. Also polyester (bondo) does not bond well with wood. In my opinion - a nice small fillet made from cheap wood flour and epoxy is the way to go.

[gonzo]

Get two pieces of plywood, glue them with bondo, and try to split them. Epoxy bonds to polyester very well too. Fillets are in compression anyway, so bonding is not so critical.

[LKJR]

I"ve been using epoxy on my runabout I"m redoing the stringers and transom in. Can I use poly mixed with the fine sawdust for the fillets and epoxy tape over that? I've ordered the epoxy it's just not going to be here till wednesday or thursday. and the poly resisn is local and fairly cheap.

the stringers are glued in now with epoxy. just no fillets or tapeing.

[gonzo]

Polyester will not bond well to epoxy. Also, sawdust is not a good material for thickening. It has too many contaminants.

[Ilan Voyager]

I worked on my first epoxy wood boat in Sweden in April 1971 as a young aprentice...Many other came after.

First of all, forget of polyester on or under epoxy. Polyester is a poor glue with a lot of gazing of styrene and its cure is inhibited at the interface by the phenol of the wood and/or the amines of the epoxy resin.

Second forget of mixing in the epoxy resin some strange things like cement (too basic= high PH), wood dust (too contaminated by dirt, CCA, preservers etc) and other powders.

The main requisites of an additive are: dry, good bonding, neutral PH, not reactive products so it won't make a chemical mess in the resin. The mix must stay stable during years, and some strange additives can transform your fillets or gluings a miserable thing after the chemical reaction has destroyen the resin.

Basically you have the collodial silica, quartzite, industrial talc, cotton flour, wood flour (cheap and effective, specially the maple sold by Raka: 75 bucks a enormous bulk), and the "artificial ones" like glass bubbles or microspheres, copolymers and so on. The "natural" ones are not expensive and why do you run in possible trouble for something that do not cost 1% of the value of the boat?

Although it's not the best additive, industrial talc costs less that 1.5 dollars a pound and can be found everywhere in the US.

[JEM]

It's worth purchasing some good woodflour. RAKA or BBC has it cheap. Buy yourself a 5lb bag and you'll have plenty for the next boat.

That or see if there's a cabinet shop near you. They probably have lots of woodflour they'd love to unload. I did that with mahogony. Stuff was pure and easy to sand when dried. Only real draw back was it made very dark fillets so if you're using light colored wood, it'll look funny.

I've heard talc is effective but heavy.

[gonzo]

For a 12' boat, Bondo works fine for fillets. It does the job and costs little. Also, it sets fast. Even though an epoxy/silica fillet is stronger and adheres better, overengineering is a sign of poor design. The material that costs less and is adequate for the purpose is best.

[JEM]

I gotta disagree. I don't think Bondo is good for that application. But different strokes for different folks.

[gonzo]

jem: what do you base your opinion? I have tested bondo and it works fine.

[JEM]

Poly and epoxy don't play nicely together. I'm saying if you've started with epoxy, then you need to stick with epoxy and fillers that work well with it. If you've started with poly, then stick with it.

Now there's lots of boat made from poly resin that are doing just fine. I just know that for the home boat builder, epoxy resin is safer, stronger, but more expensive.

Bondo is also polyester based. It's proven that epoxy is far superior with regard to mechanical properties than polyester, vinylester.

Trying not to be a purist here, but there's lots of data regarding the mechanical properties of epoxy and various fillers. My personal opinion is spending the extra bucks for epoxy is worth it.

[gonzo]

OK, but have you tested it?

[JEM]

No, I don't have the proper equipment (or desire) to conduct any meaningful tests.

Scantling/design books and ISO regulations have different lamination schedules for using epoxy and polyester. The poly scantlings are always heavier.

I'll say again, there are lots of boats made from poly that are just fine and plenty strong. But it's not safe to use poly for scantlings intended for epoxy. There is no direct 1:1 substitution.

That probably matters less and less as the boat size and intended speed decrease. Maybe on a knock-around paddle pond boat it's ok. But that stuff stinks and is nasty to work with anyway. The cost, however, is much cheaper than epoxy. Poly products much more readily available too.

My personal opinion is building a boat using bondo isn't worth the investment and risk. That's what this forum is about, personal opinions, right? ;)