If you are sure it is wood atached to the paint then it may be the
plywood. Latex exterior paints last longer on wood, oil may protect for
moisture better and on horizontal surfaces but does not last like latex.
No photo? You need to find the cause of the issue. Sherwin Williams
will send out a rep to address your issue and even lab tests can de
done. Use their product as they say and you have a warranty . You say
oil primer and paint was used, There are good oil primers and paints and
bad ones, you dont say what was used. Sherwin Williams makes some of the
best paints and as all companeis do only guarntee their Latex products.
They have one with a lifetime guarntee against peeling, but it will do
you no good and have no warranty if it is the wood or original paint
that is failing. Get onsite opinions and get Sherwin Williams out. If
you get out a paint rep , use their products as they specify exactly
then failure is their fault. I have had Sherwin Willaims pay up on
several jobs, Ive had P&L totaly back out.