epoxy hardening/filling Q's

Hi All

I mixed up a batch of epoxy the other day and glued a few but joints and then thickened the rest an used it to fill a few holes and gaps.

the thickened epoxy is rock hard, but the epoxy on the but joints still hadn't set up after 48 hours.

I'm guessing that I didn't mix it well enough before I glued the but joints and by the time I mixed in the flour for thickening it was mixed well enough, just thought I'd ask in case I'm missing something.

also, I was wondering if I need to fill countersunk screws if I'm taping over them or if they'll fill well enough when I wet-out the tape.



minno

 

You may have had a mixing problem. Give it a few days and warm temps. It may still kick, but if not be prepared to remove all of the uncured go and clean your pieces thoroughly before redoing it. You could go either way with the countersunk screws. And, are they flush or deep? That could matter also. Are they temporary screws? Best to get rid of any unneeded metal.

 

I went ahead and took them apart, they were still tacky enough that I had to scrape most of it off before I could sand it. I sanded it down to the wood so it should be ok.

I've never worked with epoxy before so I tend to rush things wanting to get it on before it's too stiff to work with, I'll stir for a couple minutes next time and see how it goes.

I set all the screws flush or just a hair deeper, they're #6 so not much to fill.

On the but blocks the screws are temporary and I'll fill the holes from both sides and tape the outside, but for the rest the plans say to leave the screws in and since it's a really light boat I figure it's a good idea.

minno

 

A thorough mix is one of the biggest mistakes novices make with epoxy. Scrape the sides and bottom of the container and insure it's well mixed. Mix ratio is another problem for the newbie. Unlike other resin systems, epoxy needs to be pretty precise, within a couple of percent or it'll cause curing problems. The taller the mix ratio, the more true this is. In other words on a 2:1 mix you can be off a lot more than a 5:1 mix will tolerate.

 

Minno, I'm new to epoxy, too, and have learned a few things (some of them the hard way!), but I agree on the mixing: Get a timer app and mix for a solid three minutes. That helped me a lot.

Also, I have learned that really thin layers or applications can take (a lot) longer to kick and set up than thick applications. Additives like milled fibers, silica or wood flour seem to speed up kicking quite a bit and you have to work fast when you use them.

Make sure the surface - not just the air temp - is warm and preheat it (safely!) with a heater, if need be, that way the exothermic reaction isn't sucked away by the cool temp of the structure.

Good luck!

Curtis

 

3 minutes is a bit excessive, but will insure it gets mixed. It's mixing action and technique that's the key. Wipe every square inch of interior surface in the mixing cup, to insure its all mixed. I usually can mix 6 ounce batches in a bout a minute. Applying thick layers isn't typically recommend, unless it's a fairing compound. Temperature is the real game changer, when trying to get goo to kick off quickly. Every 18 minutes above 78 (F) will half the cure time. So, some heaters in a home made tarp tent over the work, can easily get the inside to 114 degrees, which will cut the cure time by 2/3's. I regularly can "cook" epoxy in the 125 degree range with a simple tent and some insulating blankets. Lastly, don't cook the initial coats on raw wood, as it'll just outgas and cause problems.

 

thanks all for the advice, after measuring very carefully, mixing thoroughly, and bringing the job into the house to stay warm I taped a couple chines yesterday, the one I did first is still quite tacky, the one I did second is rock hard, they were both done with the same batch of epoxy, is that normal?

minno

 

The same batch of goo should cure the at same rate, assuming the temps are the same.

 

That's likely the answer then, the tape that took longer was about 10" closer to a cold concrete floor, next time I'll use a couple step ladders to rig up a bench as high up as I can where it's warmest.

Thanks

minno

 

If the mix goes off OK, it will be good wherever you have used it. The setting time is somewhat related to the thickness of the coat (as well) - so very thin sheathing takes a little longer than a large fillet (with fillers). Just allow a little more time or as you have found, keep the temperature up a little.

Key is thorough mixing and starting the exothermic reaction.

 

All above looks like good advice. I have just started using epoxy to build a small boat and had the same problems as the OP. I was advised to keep the joint warm especially during the first few hours to kick off the exothermic reaction. Also, a bit of wood flour to thicken epoxy in joints seemed to help; possibly the filler allows a slightly thicker joint which seems to cure more reliably than a very thin one.

 

The filler doesn't affect the cure rate, though because thickened goo tends to be applied in heavier batches, compared to a simple coating (for example), it cures faster. The thermolytic reaction is going to happen, no matter what you do, once the two elements of the mix are combined. Even at very cold temperatures this reaction will begin, though may occur very slowly toward a cure, because of the low temperature. All epoxies will have a hardener that has an ideal temperature range to work within. Stray very much on either side of these ranges and it'll kick off way too fast or can take months to fully cure, depending on which side of the range the ambient temperatures are. Applying heat after the mix is applied can cause issues with raw wood. It's best to bring the work up to desired temperatures, before applying the goo, then keep it at this temperature for several hours. A simple heater and a tarp, blanket, etc. will great contain heat, if used as a tent of work piece cover, during the cure (with a heater). One thing I use a lot in the winter are panels of 1/2" foam insulation, with a foil face on one side. Place the foil face toward the heat, throw a blanket over the whole shooting match and you can cook a piece with a small heater.

 

I'm fortunate enough to have a spare room with electric heat that I can use to keep everything warm.

do fillets have to be taped?
I've got a few gaps that I'd like to fill with thickened epoxy and leave a small fillet on that don't really need to be taped otherwise.

minno

 

It depends. My tests (and others) suggest taping isn't as important as it seems, but some loads are transmitted better with the presence of tape.

What is it you think you need tape on, how heavily loaded, how big are the gaps, etc? If it's not highly loaded or submerged, just a structural fillet over the gaps (silica, milled fibers, wood flour) and call it a day. If the gap is huge, maybe on the chine seam or transom corner, you'll probably want some tape.

 

It's just crack filling on an inside bulkhead against the hull, the hull is 4mm Luan so pretty flexible, I'd like to use a small, say 1/4 inch fillet to seal all the corners of the floatation chambers against the hull as well, not much flex there.
Hmm, wonder if I can use silicone for inside the flotation chambers?

I'll be taping all chines (stitch and glue), stem, and the transom inside and out and using about 1/2" fillets on the chines although the plans don't call for them, more for looks than anything structural, I don't want the stitching wires to show and I'm concerned about voids under the tape where the wires are.

minno