I'd advise against applying a resin coating before bending.
1. It might reduce the bending radius for the plywood which might not
then bend to the curvature of the hull. It will increase the effort in
bending, how much I do not know.
2. Bending will put many tiny cracks in the resin letting in water.
I've seen a nice wooden kayak with a thin interior coat of epoxy which
was left out all winter. Water worked it's way into the wood under the
epoxy. That's extreme.

I don't see any problem painting or oiling the inside surfaces. I don't
think bending would crack latex paint or linseed oil. I use linseed oil
on the intror of my small boats but white paint would lighten up the
interior of a Weekender's cabin.

Waterproofing might not be much of an issue. It's doubtfull the
interior of the boat would be immersed for any length of time. Maybe
water lying in the bilges. It would be pretty easy to maintin a finish
on the inside bottom of the boat.

I don't see any problem with polyester resin. The plywood needs to be
clean, and lauan (RONA sells a nice meranti underlayment now instead of
the wormy lauan they used to have) is porous and tends to soak up resin
nicely. Adhesion should not be a problem. I use polyester to seal the
chines on my underlayment lauan and virola (now there's a *really*
crappy plywood) boats.

Andrew Butchart wrote:
For my current project - a Stevenson's Weekender - I'm very close to being
able to put on the sides. To avoid crawling around inside the boat coating
and painting, I'm wondering about doing that before I put the sides on.
I'll mask off where the glue joints will be during a trial fit. The sides
will be 1/4" luan coated with polyester resin (no fiberglass on the inside).
I'll fiberglass the outside after assembly.

My concern is whether the resin will be able to take the bending after it
sets up, or will it crack? Has anyone else tried to do things this way? I
don't really want to crawl around inside the cabin breathing in fumes trying
to do the coating.

--
Andrew Butchart