How large of a epoxy batch?

[MichaelG]

How large for the first batch of U.S. Composites epoxy resin with medium hardner going on 7.5 oz. e-glass cloth on a 16' x 4' x 1/4" lauan boat bottom. (I'm putting on 3 layers). I'm worried if I mix enough to do it all, that I won't finish job before the batch starts to set up. Do I have to do it all at once or can this be sectioned? (I would like to do it all at once.)

MichaelG

[KnottyBuoyz]

Hey Michael

I think you'll want to do it in smaller batches. Two sets of hands is preferable, one mixing and one wetting out the glass. Big batches heat up quickly and kick off too soon. Smaller batches will lead to less waste. I haven't done anything near that size (yet) but this is the advice I've been given.

Under ideal conditions you'll use about 55 ounces of goo for a panel that size.

One way to do it all at once, if you have the room to work is lay out your glass on a table covered with plastic (visqueen) or something similar. Wet out your glass on the table then once it's fully wetted out roll it up on a dowel or something similar. Move it to the boat and unroll it where you'll squeegee it onto the boat.

Plenty of different ways to skin a catfish! ;-)

[chandler]

You might be able to pull that off if you mix the epoxy in a couple of roller trays and put the trays in the fridge or freezer while laying the glass.
The problem with large batches is the pot life, once it starts to go off you're done, it will get so hot you'll swear it's gonna self combust.

[Pericles]

MichaelG

The difference between pot life and working time with epoxies can cause misunderstandings. Pot life is a standard test, common to all manufacturers of epoxies and determines the speed of the hardeners. An example would be West 105 resin and 205 standard hardener. correctly dispensed at 5 : 1 ratio and thoroughly stirred. The 100 g sample in the mixing cup will go off in 6 1/2 minutes at 25 degrees Celsius. At 18 degrees Celsius pot life is extended to nearer 22 minutes. With the same resin and 206 slow hardener, pot life at 25 degrees Celsius extends to 23 minutes and at 18 degrees Celsius, nearly 40 minutes.

However, the working life depends on what is done with the mixed epoxy. If it is poured into the roller pan the heat generated is dispersed and the epoxy says workable for much longer than the pot life would suggest. The viscosity of the epoxy is affected by heat, the warmer. the runnier! The greatest release of heat comes at the moment of gel and the thicker the film of epoxy, the runnier it is. This fact explains the mystery of runs. If too much epoxy is flow coated onto the glass cloth in an effort to fill the texture, pools of epoxy can form where the glass cloth has slightly floated off the wood surface below and they are not easy to see. The applicator goes for a break, the pools of epoxy heat up more than the surrounding areas and the runs form and then go off more quickly. Bugger!

The answer is more thin coats applied, wet on wet, as the underlying coat tacks off. Therefore, a slow hardener approach, can be counter productive, as the waiting time between thin coats is too long.

Hope this helps,

Pericles

[FAST FRED]

If you are working on a mostly flat surface , where the mix cant run off , many times it is easy to simply pour 75% on the surface at once.

Then spread it in and use the remainder for any dry areas.

All the epoxy has a limited work time , but by spreading it thin from the start its own exotherm wont cook the batch sooner.

Its not as messy as you think.

The joy of epoxy is the layers bond to each other with 100% strength.

So a second fill area is not a horror.

FF

[MichaelG]

Fourm

Thanks guys, the info helps.

MichaelG