It was about 85 degrees today. I decided to mix up some epoxy filleting goo to fill screw holes and fillet some seams. I used Marinepoxy with slow hardener. For 3 ounces of epoxy/hardener mix I added 2-1/2 ounces (by volume) each of silica and wood flour. The thickened epoxy had the consistency of peanut butter, but about an hour after application it had started to sag fairly badly. What is the ideal formula for thickened epoxy for this use? Paul, any comments? My inclinantion is to use about twice as much silica as wood flour to thicken the epoxy; maybe 4 ounces by volume of silica to 2 ounces of wood flour.
Dave
As a point of interest, I've been using baby powder to obtain a thick consistency and it has worked perfectly, no sagging and very easy to handle, excellent results.
Interesting, Hershel. I may experiment with the idea. Silica is supposed to be the stiffener of choice for epoxy, but is hard to sand. I'll be interested in Paul's comments about baby powder as well as silica/wood flour.
Dave
Dave, The baby powder is very easy to sand. I don't measure the amount. I just keep adding the powder and mixing until I get the consistency that I want.
My Vacationer is just about finished. See picture under my "Dolphin Striker" post.
I hope to launch in the next 2 weeks. I just have to put the hardware on the spars.
Talc (baby powder) is a very effective thixotropic additive, though lacks the strength of silica. I use talc all the time, but also silica and other stuff in each mixture I make.
Because you're using it with wood flour, the talc is helping smooth the mixture as well as thicken it up. Each goo mix needs a few different things. A bulking agent (balloons, spheres, etc.), a strength agent (wood flour, milled fibers, graphite, aluminum oxide, 'glass strands, stone, etc.) and a thixotropic agent, which stiffens the mixture to the viscosity you need. Silica is the usual choice for this, but there are other things that work and make sanding much easier.
Most every mixture I make will have some silica in it (again to control viscosity). I only use talc when I need to sand the finished product. I also use cooking flour (wheat, corn, rice, whatever) as a bulking and strengthening agent too, as well as all sorts of materials in goo. It's cheap and the other half usually can't tell when I've raided her baking stash.
What I'm trying to say, is each mixture is application dependent. Filleting a bulkhead in place, in a location where the tabbing (tape) will never be seen, well, I don't bother with talc, because I'll make the fillet, cover it with tape and call it good. On the other hand, if the bulkhead seam, where it lands in the cockpit (for example) will be visible or need to shed water neatly, I'll take special care to do a nice job of it, use a bit of talc to make smoothing and sanding easier, so the end result looks good, when a perfectionist comes aboard and has a look at my seams.
Hershel: What formula do you use to get goo that will not sag on a vertical surface?
Dave
Dave, I use 2 to 1 epoxy that I bought from Graham (B+B Yachts) and Rexall Baby Powder which I don't measure but just keep adding powder until I can barely stir it. I could probably make mast hoops out of it as it is that thick.
I should add that wood flour can "release" some resin as the exotherm starts getting going. I've recently discovered this aspect of wood flour and now don't recommend it for a thickening agent, unless it's commercially purchased, a hardwood and very fine. The stuff out of a belt sander (or any sander) isn't fine enough, makes a lumpy fillet and the particulates are too large, so the resin tends to drop out once it's applied and the heat builds up in the fillet/joint (making the resin thinner so it runs). I do use wood flour to match colors when making repairs, but only hardwoods are used, as softwoods seem more susceptible to this.
About the proper mix, use the filler (bulking agent) of choice, depending on what you want (heavy structural fillet, light structural fillet, cosmetic fillet/fairing compound, etc.), then use silica to control viscosity, so it doesn't sag or run out of a fillet or on vertical or overhead surfaces. On vertical and overhead surfaces, you'll need a peanut butter consistency. The right viscosity will cause a hunk of epoxy to remain standing if you pull a putty knife up through it. The little mini mountain you make, as the knife is pulled up, will leave a little point on top of the goo pile. If the little point remains standing, it's thick enough. If it flops over and sinks down, back into the pile, it needs more silica. A little silica goes a long way, so add a little at a time and test it. You'll get a feel for this with some practice.
That is good to know paul. I've used quite a bit of spruce wood powder from my belt sander when I built my Vacationer.
I figured it was better than bondo. Now I know for next time.
By the way, has anybody heard from Dave Blake. He was building a really nice Vacationer and has not been on the board for a very long time.
He's not been active for quite a while. Lets hope he's so buried in completing or sailing his Vacationer, he hasn't the time to play with us.
I still use wood flour for things, but mostly to match colors. For example on a glued lap build, I used milled fibers and silica mostly in the seams, but a pinch of Meranti or Okoume (depending on which plank) in the mix made the goo look like what it was going on. Not really a big deal under paint, but under varnish, much better than just a milky white silica ladened joint.
For example, just below the splash guard (slightly aft of it), there's a seam in the varnished sheer strake on this boat. The joint was milled fibers and silica, with a touch of wood flour for color. Lousy picture, but in real life, you had to get within 12" to see the joint.
Another thing wood flour is good for is holding a joint apart, while the goo is curing. It's really easy to mash all the goo out of a joint if you need to use a good bit of pressure to make the joint close up neatly. This starves the joint of adhesive, but if you use wood flour or especially wood dust from a sander, the particulates are big enough to hold the joint open a bit, so the epoxy doesn't get crushed out. In other words, don't give up on it, just remember there's some limitations.