Re: Hard, Fast or Slow? Fast, usually

From: J.R. Seven (seven@mechsys2.me.berkeley.edu-DeleteThis.com)
Date: Wed May 09 2001 - 10:51:03 PDT

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Date: Wed, 09 May 2001 10:51:03 -0700
From: "J.R. Seven" <seven@mechsys2.me.berkeley.edu-DeleteThis.com>
Subject: Re: Hard, Fast or Slow?  Fast, usually
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Ed Scott wrote:
>
> For anyone using the West System, a query:
>
> On cool California evenings during the summer, do you get better results
> with the fast hardener or the slow stuff. Seems like the slow stuff is only
> good for daytime use in direct sunlight. The stuff doesn't kick sufficient
> for ding repairs at night (55-70 degrees) for 25-30 minutes. If I apply it
> to dings before then it runs all over the place, and that's when metering a
> tad more than the pump puts out.
>
> Btw, any downside to using a little bit more hardener? I understand it
> might make the finished product brittle. Also, when changing hardeners, any
> concern about or procedures for cleaning the pump?
>
> So, for best results to get hard do you like it fast or slow? :-)

I've been using the fast stuff for repairs with good success, at night and in
the daytime in a shady area. Occasionally I have to mix a second batch because
I couldn't finish the job before the first jelled up. The most significant
problem has occurred when I mixed some up with microspheres (for use as a dent
filler and rail shape adjuster) and left it in the mixing pot a little too long.
The filler acted as an insulator, and the stuff was putting out smoke in a few
minutes as it melted the bottom out of the plastic pot.

I think the slow stuff is mostly used for larger jobs which take more time,
such as laying up full-sized laminates rather than small repairs.

As for ding repairs where some strength is needed, you want some glass in there,
not just plastic. So it's best to use the stuff when it's thin (to wet the
glass) and keep it in the right place with a mold made from tape or something.
Where you don't need strength, mixing in some filler such as the aforementioned
microspheres keeps the stuff manageable.

Everything I've read says that varying the ratio of hardener to epoxy is a bad
idea. This works for polyester because polyester hardener catalyzes a reaction
in the polyester resin itself, and more catalyst makes the resin change more
quickly. But with epoxy, the final hardened plastic consists of chemicals from
both the resin and hardener; if some of either one is missing, you'll get a
percentage of unhardened stuff in the resulting plastic, thus weakening the
final product. (Any chemists like to give a more precise description? I know
this one isn't exactly rigorous.) When I've messed up the ratio, I got something
which had the stiffness of rubber but cracked easily as well.

I wonder if people who live in the Delta (or farther East) need the slow stuff?

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