epoxy question

[markfitz]

We have finally flipped the weekender upside down, and have mixed our first batch of epoxy. We started out filling screw holes using maple flour as an additive, and that worked well. Nice texture, good work time, very spreadable. Next we wanted to do the fillet between the keel and bottom, and we had purchased fumed silica for this part. A few questions about that. It was hard to mix, I'm sure it's dangerous to breathe - (it floated around like dust motes) it set up FAST, and sagged when we put it on. So...my question are -- why did we have these types of problems, and is it OK to just use the maple flour instead, since it is so much easier to work with? (Please say yes, because that's what we did. It would be impossible to rip it out of there now.....) What is supposed to be "better" about fumed silica as a filler?

[markfitz]

I didn't think about the possibility of adding BOTH. Duh...also, it seems like even using the slow hardener, the work time is shorter than I would have thought. Also, one more question if I may -- the directions that came with the epoxy say to put a coat of just resin on the wood first, then put the cloth, then more resin. Obstensibly, this is to prevent or lessen "dry spots" when applying the resin over the cloth. Any benefit to doing it this way? What do most people do? Thanks!

[mustangermatt]

One real advantage to "wetting out" the ply first is it gives the epoxy a chance to be absorbed by the wood. If you don't do this you run a high risk of having starved areas where the wood sucks the epoxy right out of the glass from underneath. I don't let the wetting out coat harden, I just let it sit there for 5- 10 minutes before I lay the glass out.

[warthog5]

All the mixing, short work time and consisty of batchs is why I quit using

epoxy with cabisol mixed into it.

I have been using FGCI's 2 to 1 Epoxy bonding putty. It comes in a paste for part A and a thicker than Honey for part B. I weigh it out on a postal scale set to Grams. It's mixed together like you would Bondo, folding it over onto itself.

It's a breeze to work with. No slumping, lays down like butter.

http://www.fgci.com/

Part A 65406084 EPOXY BONDING PUTTY,BASE-QT QT 11.95

Part B 65606084 EPOXY BONDING PUTTY,ACT-QT QT 11.95

Don't forget to order 2qts of Part A as it's a 2 to 1 mix.

[Frank Hagan]

What's the word they use for the way epoxy cures and generates heat, "exothermic"? Something like that.

One thing I found when mixing either wood flour or fumed silica into the mixed epoxy was that you don't want to cover the batch of epoxy with an insulating layer of "mixing" stuff. If you insulate the top of it, the heat being generated can't get out off the top, and it really accellerates the reaction. So now I use pretty large mixing containers compared to the batch I'm mixing, so there's a lot of surface area for heat to dissipate. In really hot climates, people use a tub of ice water and set the mixing container down in it to retard the reaction (or at least prevent it from accellerating!)

I also had better luck with wood flour than silica, and now mix a bit of silica in, but use mostly wood flour. Talc has been used for a long time as an additive, but I don't know how it works. The maple wood flour I've used mixes to a dark, mahogany color, while pine wood flour matches the honey oak color.

[RussMartin]

What's the word they use for the way epoxy cures and generates heat' date=' "exothermic"? Something like that.[/quote']

Exothermic is indeed the correct word, Frank.

[Craig]

Talc works like a champ Frank. I get it by the pound in cans at the dollar store. It is very easy to control the mix and it cures up like a rock but sands quite easily with power tools or by habnd if it is a small area. I have even used Saralees old face power in extremis

[capt jake]

Now I have always been told NOT to use talc as it has no strength.

Talc is the thickening agent in most 'Bondo' type products; and 'Bondo' is supposed to be a no-no. :?:

I personally have had pretty good luck with fumed silica, though it is a buggar to get completely 'wet' in the mix.

[Frank Hagan]

Talc is a filler, so the strength of it isn't an issue (epoxy is plenty strong). I think the issue with talc, like wood flour, is that it is "hydroscopic" and will soak up water. In Bondo, the amount of hydroscopic filler is so high that the resin itself falls apart as the filler takes on water. In an epoxy mix, you have so much more epoxy than filler that you don't run the risk of the filler taking up the water. The epoxy encapsulates the filler, so to speak.