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Re: plywood capacitor box


From: 	Peter Electric[SMTP:elekessy-at-macquarie.matra-dot-com.au]
Reply To: 	elekessy-at-macquarie.matra-dot-com.au
Sent: 	Monday, July 14, 1997 1:45 AM
To: 	Tesla List
Subject: 	Re: plywood capacitor box

Tesla List wrote:
> 
> From:   Michael Smith[SMTP:md.smith-at-usa-dot-net]
> Reply To:       md.smith-at-usa-dot-net
> Sent:   Friday, July 11, 1997 3:07 PM
> To:     tesla list
> Subject:        plywood capacitor box
> 
> greetings all,
> 
> Nice to see the list up and working again...The shakes are
> starting to go away now.
> 
> I have a quick question. Can anyone recommend a very good and
> relatively inexpensive coating, like a two part epoxy paint or
> something that would be able to seal up the inside of a plywood
> box used to a capacitor case? I would like to avoid building a
> plastic box if I can.
> 
> I would like to use several coats of polyurethane, then coat the
> inside with something to hold in the oil.
> 
> Anyone have any experience along these lines?
> 
> Michael Smith
> md.smith-at-usa-dot-net

After several years of experience building plywood boats, I have
discovered several things about wood.

It's main problem is that it is not dimensionally stable, i.e. it
changes shape and thickness with changes in relative humidity. Its
coefficient of thermal expansion is also very different to any kind of
plastic coating. The upshot of this is that if you simply coat the wood
with varnish or resin, the coating will develop hairline fractures over
time and your oil will soak into the wood. The only way to do this
sucessfully is to use fibreglass re-inforcing mesh in the plastic to
give it strength.

It is best to use epoxy (two pack) resin as this binds to wood much
better than polyester and will need less layers of fibreglass. Coat the
wood with one layer of resin. When the layer has set but is still a bit
sticky, put down one layer of medium weight, chopped strand mat
(available from fibreglass repair shops) and thouroughly soak this in
resin. When this has just set, put down a last layer of straight resin.

I guarantee that if you do this, your box will not leak. Hope this
helps.

Cheers,

Peter E.