Poly or Varnish for Exterior Wood Project

I am starting to build some storage boxes out of exterior ply and trimming it out in redwood. They will be fitted on our utility trailer so they will be exposed to all the elements. I will more than likely be spraying the finish on. I was just curious which finish will be a harder more durable finish for exterior use. Any suggestions? Thanks in advance!
Rick
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RampRat wrote:

Without question, marine finishes will be the best, if you are willing to pay the price.
If so, I would shoot some high build epoxy primer followed by a good marine enamel.
HTH
lew
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I vote for Marine finishes as well -- if you live in a climate where salt is used on the road during winter driving, otherwise, maybe nothing is simpler.
Lew Hodgett wrote:

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Will
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Maybe a marine-grade spar varnish? Takes awhile to dry but more water resistant than other finishes.
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wch wrote:

Not really.
A good spar varnish never really gets hard.
That is why is is used on wooden spars that tend to flex when placed under load by the sails while sailing.
For most other applications, true spar varnish is a total PITA.
Basic reason I suggested epoxy high build primer is that it will seal the plywood and prepare the surface to receive any number of finishes.
The lowest cost one being enamel.
Just guessing, but can't think of any reason to finish exterior plywood bright (clear varnish). Especially if a structrual garde has been used.
HTH
Lew
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Lew Hodgett wrote:

I use spar varnish on wooden signs outdoors all the time. Worked good for me so far.
--RC
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Keep in mind that the best one will probably have to be redone every 3 or 4 years.
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