On Wed, Aug 5, 2009 at 11:02 AM, Karl Vacek <kvacek@ameritech.net> wrote:
> These just came through the MG-TABC list and looked really helpful, so I
> thought I'd pass them on. I never considered the wet sanding technique to
> avoid flash oxidation on aluminum, etc., but it sure makes sense.
>
>
>
> I certainly agree that moat hardware store epoxies aren't worth the goo in
> the tubes!. For many years we had garage customers come by our sailing
> hardware store looking for JB Weld or a sub. to close pinholes in gas tanks.
> By & by we switched them over to W.E.S.T. epoxy as a better product.
> Basically you clean leak pinhole with whatever, then force epoxy thru hole
> (enlarging if necessary). This creates a "mushroom" head in tank that seems
> to produce a reliable fix. The same technique can be used to patch pinholes
> in air toys, rafts, etc. Only instead of using epoxy use 3M 5200 adhesive
> sealant. Force it thru to make a mushroom head on inside of vinyl & you are
> back in business.
>
WEST's stuff is great; however, it's very expensive, hard to find if you
don't live somewhere they build boats (or airplanes) and I'm pretty sure the
smallest size sold is a quart. There are other good epoxies available in
smaller quantiies.
Incidentally, the basic problem with the hardware store epoxies is that
they're made to be easy to use. The ratio of resin to hardener in pure
epoxy systems is an inconvenient number (about 2.3 parts resin to one part
hardener; varies depending on which hardener in use, and whether it's spec
as mass or volume). There are three basic ways to deal with the problem.
First is just to live with it, which is what industrial users do. Second
is a variation of method one, using special dispensers or packaging. WEST
use special pumps that are calibrated (in theory...) to provide the right
ratio. The third is to add a filler. Some of the best products use resin
as a filler in the hardener (these are called "adduct epoxies"; they've got
shorter pot life, faster curing, and less (or no) amine blush. There are
any number of other possible additives. Some serve useful purposes (adding
UV resistance, changing viscosity, pigments, mechanical properties (I use a
metal filled epoxy for a number of things), suitability as a dessert
topping), others are just there because they're cheap. Hardware store
brands tend towards "there because they're cheaper than actual epoxy".
--
David Scheidt
dmscheidt@gmail.com
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