Thread: Epoxy troubles

Originally Posted by hereselmo1

By the way do I need to allow for the density difference between the epoxy and the hardner or do I just mix it 2:1 by weight?

Well we're getting past mere "pedantic" and into the realm of "geeky" here now.
There are various data sheets available for these resin systems. The safety data also gives density and indications of the chemical composition (which can be handy if you want to assess whether a relatively unknown brand is using similar technology to the big names).

FGI (the brand I use) gives SG figures of 1.1 for the resin and .97 - 1.0 for standard hardener, 0.95 for slow hardener. This means there is a bit over 10% difference and it would be worth using that bit less of hardener when measuring by weight.

West quote the resin SG at 1.15 with different ranges for their hardeners: 205 = 1.04 SG, 206 and 207 = 1.0 SG, 209 = 0.98 SG. They recommend a mix ratio for 205 and 206 of 1:5 for both volume and weight methods. However for the 207 and 209 they recommend 1:3 by volume and 1:3.5 by weight.

I doubt this would affect performance significantly but someone using the volume ratios when measuring by weight could well find they always have resin left over in the bottle when they need to buy more hardener. It's a minor economic penalty.

The MSDS for Bote Cote only quotes one SG figure for the system: 1.12. If there is a need for different ratios between vol/weight then it should be mentioned in documentation provided when you bought the stuff. (...and if you're like me all you have to do now is dig the instructions out of the bin).

One final thing about mixing small batches: I put the hardener in first. Why? Because it is easy to put in too much when pouring from the bottle ('cos I'm too much of a cheapskate to buy pumps). By taking note of the exact amount I can then put in an appropriate amount of resin. If I go over by a gram on the resin it doesn't matter as much as going over by that much with the hardener (remember the 1:5 ratio!). It always seems to mix OK as long as I remember to mix it at all.

Starting to get that deja vu feeling - have I posted this crap before? Anyhow that's enough geek for tonight - now you can forget it all and just get on with boat building .

Most epoxy spec sheets list the ratio by volume and weight, since the densities of the resin and hardener are different.

My epoxy ratio (System3) is 2:1 by volume and 100:44 (I think) by weight, printed right on the bottle.

Seems at least interesting that System Three elected to use 100:44 when 25:11 would have been a little easier to work with.

I read the System3 bottle again yesterday evening ... it says 2:1 or 100:50 by volume and 100:44 by weight. I suppose people get less confused by parts per 100 compared to parts per 25 when measuring on the scale.

I started daydreaming and thinking about this thread when I was mixing a large cup of epoxy last night! After I finished stirring, I couldn't even remember if I mixed in the hardener! I decided to take a chance and just apply it on the boat. Luckily this morning the epoxy is hardening up. Whew.

This is a common issue with repetitive tasks. You'll find it's very difficult to remember if you use a turn signal, when you pulled into you street. You think you should have, but when pressed to actually remember the task you can't, unless you were spilling coffee in your lap at the time. Most of these things become "muscle memory" and your brain stops participating so much in the process and lets you body "remember" how to do it. This neat little trick of human behavior, permits the brain to focus on more important things and is a primary reason we can multi task.

This is brilliant. Not only am I picking up lots of info re' epoxy mixes and boat building, but also excuses I can use for my fading short term memory.

Originally Posted by PAR

This is a common issue with repetitive tasks. You'll find it's very difficult to remember if you use a turn signal, when you pulled into you street. You think you should have, but when pressed to actually remember the task you can't, unless you were spilling coffee in your lap at the time. Most of these things become "muscle memory" and your brain stops participating so much in the process and lets you body "remember" how to do it. This neat little trick of human behavior, permits the brain to focus on more important things and is a primary reason we can multi task.

Exactly .. and if wanting to accomplish higher level tasks like being a good sailor or piano player this needs to be developed.

I ran into the idea reading "Sail, Race and Win" by Eric Twiname - who writes marvellous books on improving your sailing - or wrote - he died quite young.

It is one of the few books that can improve both your sailing and your piano playing at one reading. Possibly helps with tennis too.

MIK

Sailing blind folded is also a great teacher. I use to do this a lot as well as moonless night sailing. It forces other senses to pick up on the subtle clues of sail trim, boat speed, wind pressure and direction, etc.

Twiname mentions this ... also he suggests trying things during races to see the effect and also to build skills. For example sailing the whole race with windward heel.

The point is to learn things that improve your future results rather than help you win the current race.

Also doing practice out of races doing things like slow spinnaker hoisting or tacking to work out where the slow bits are.

His books are a lot of fun too.

MIK

Checked the epoxy this morning, hard as a rock. Might be OK after all

As always, I'm a couple of days late...but here goes:

• Epoxies are pretty fussy about the mix ratio. If in doubt it is better to err on the side of less hardener in the mix. Apparently the strength of the bonds is affected less having slightly less hardener, than having too much.
  
• If a mix is taking it's time to go off (ie it gels but is soft) - mild heat to get the reaction going can do wonders. Not too hot, but if you can get a surface temp between 60-80 deg C you should be ok.
  
• Patience pays dividends if the mix isn't 100%. It'll go off in most cases, but because of the incorrect mix, it may just take longer.
  
• If it is really bad, and early enough (still jelly) - acetone, and lots of it.

I don't know if it makes me qualified to give the above "advice", but I worked in resins research for a Composite Fibre facility at the University, and talked a lot with the chief chemical engineer (who made some awesome stuff). I also did Lloyds testing of laminate samples...

Howdy,

Spot on. Only thing is that I would stress that if the epoxy doesn't go off it is almost always a case of not measuring properly or not mixing properly.

You can't trust the epoxy to compensate for sloppy procedure - though as per the third point - sometimes you get away with it when you shouldn't ... PHEW!

MIK