First: If it is possible to return the marine plywood you'll save some
money and some trouble later. There is nothing special about marine plywood
except that the bad spots in the inner plys are filled where in conventional
plywood they are not. Marine plywood will rot just as fast as conventional
plywood. I ALWAYS use treated plywood for floor and treated solid wood for
frame and stringer replacement, then I only have to do the job once.

Second: The treated wood must be completely dry before using it. If it is
damp as it often is when purchased the polyester resin or epoxy resin will
to "take" to it, so allow it to sun dry for a period to be certain that it
is quite dry.

The frames and stringers need to be secured with both fiberglass cloth and
matt material to make them an integral part of the boat..

Third: It is possible that a sag has developed in your boat from setting on
the trailer. This is generally refereed to as a "hook" in the bottom and
will reduce the speed of your boat. During your work the boat should be
suspended by the bow and transom so the sag can be removed and try to
develop a "rocker" or "bulge" rather than a "hook". Sometimes it may be
necessary to place a plank under the gunnels across the width of the boat
and use a hydraulic jack to force the bottom back down in place until the
new frames and stringers are glassed in and cured. A straight edge
lengthwise along the outside of the bottom may help you decide how much
"hook" is in the bottom of the boat.

Don Dando