"J. Clarke" wrote in message
in.local...
In article ,
says...

Awl --

I'm looking to make a 4 ft x 6 ft base for some apparatus, out of 1/2"
plywood, for outdoor use.
It works well indoors, but I'm wondering if I can make it *truly*
waterproof
for outdoor use, with enough coats of poly, epoxy, or some clear plastic
coating-type finishes I've seen.

Not that familiar with wood/coatings, beyond having done my floors with a
water-based poly+catalyst, with mixed results.

Also, I know there's HD 1/2 plywood, which you can break across your
knee,
and there is real 1/2 ply, from a lumber yard. Are there even
harder/stiffer grades? I'm not necessarily looking for furniture-grade
plywood, but maybe that is indeed the stiffest. Cost, bang for the buck
is
a factor.
Maybe other "engineered products"?

Appreciate all input.

I didn't see anybody asking questions, but there are some that are
relevant.

Swingman did!! See my first response to him, but I'll address your
specifics as well.


When you say "outdoor use", that covers a lot of territory.

Are you going to be putting it on smooth pavemement, a well tended lawn,
an irregular surface, a beach, rocks, or what?

All of the above.
Altho, for truly irregular surfaces, they would have to be evened out
somewhat, bec altho my intended base is rigid enough for the "use stresses",
that is sort of predicated on a uniform surface, altho that surface could be
anything.
Of course, if I really beefed up the base, the surface wouldn't need to be
regular, but that would bump up the $$


How long will it be exposed? Is this something you're going to pull out
once in a while and then put back, or is it going to stay outside
permanently? If permanently, how long does it have to last? A day? A
week? A month? A year? A decade? All eternity?

Ideally, an eternity, permanent outside exposure.
Practically, 10 years.
An all aluminum/SS version would meet this, but it starts getting heavy,
expensive.


Does it need to be portable? If so, how portable? A five year old can
lift it and carry it away? An adult? Two adults? Four adults? A
lift-gate truck? A crane?

Total weight, 100#, give or take. Thus it can be shoved around, with not
much drama, but not by a 5 y.o.


An ordinary sheet of Home Depot exterior plywood (it's much like other
standard-graded exterior plywood--if you can break one piece of half-
inch douglas-fir ply across you knee you can break all of them) without
any protection should be able to survive outdoors in ground contact for
several years--how many is going to depend on the environment--it will
be longer in Death Valley than in the Everglades or Seattle but it's not
something I'd try to quantify beyond "several years".

Pressure treated plywood (should be able to find that at Home Depot as
well) will last longer obviously.

Painting it will prolong its life even more. As a general rule you want
a pigmented exterior coating--clear polyurethane and other such are
remarkable products but no clear coating is going to hold up as well
under UV exposure outdoors as a pigmented one. Follow the
manufacturer's recommendations for primer and whatnot.

Hardwood ply will be stronger but finding it with exterior glue may be
problematical. Marine plywood is a very, very high grade, but it's not
cheap and may be overkill for your needs.

Doing it "right" you would use ground-contact rated lumber for the
actual ground contact, put some pressure treated stringers on top of
that, and put your ply on top of the stringers, so it gets some air
circulation and won't be wet all the time, and paint it all with a good
grade of house paint (or you could go hog wild with Awlgrip or another
2K marine coating but that's serious overkill). That's going to be
heavy and bulky though and for something that you want to be easily
portable and that doesn't have to last very long would be excessive.

Iow, if the base is 4 x 6, use mebbe three 6' ground-contact 2x4s (or
thinner?), four or five 4' stringers across those (1x2?), and then the ply?
I think the weight will come within range, and all those cross member+ply
would certainly add to the rigidity.
Food for thought.
--
EA