I have a relatively new P-19 - a '97 model. I've yet to find a leak, even
after having the boat get rained on for several days. Most of the owners
reporting leaks seem to have older boats that were made at a time when
the attention to such details was lacking to some extent. Remember also
that older boats, after years of rig stress and other wear, sometimes
develop leaks when the old sealant fails.
I doubt you'll have much to worry about, but a tube of sealant should be
a basic item in any skipper's repair kit.
With regard to backing plates, IM used fender washers on my boat, which
seem more than adequate.
-- Eric L. Pederson P-19 #970, Necessity Bloomington, MN