On Tue, 30 Mar 2010 19:35:01 +0000 (UTC), Al 1953 wrote:

I purchased tongue & grooved redwood floorboarding (22mm x 135mm) to be
exact.

Yipes, I hope you've thought about the weight and the hinges to
support them...

For the bracing' I'm thinking three equally-spaced horizontals with
diagonals in-between. I am planning to have no vertical bracing at the
edges, as the tongu+grooved timber is nearly an inch thick, and seems
very rigid,

My gut feeling is that won't work, or not work as well as diagonal
across the corners of a box. I can't pin down why I feel it won't
work, other than "no triangle". I could be wrong, the fact that you
have 3/4" boards might be enough but they will slide past each other.

I can always add vertcals later.

The horizontals really need to be mortised into the verticals and
possibly the diagonal braces jointed in as well.

thinking I should glue the grooves before tapping in the tongues, I
assum this is correct?

I wouldn't the boards will want move and if glued they can't. I
wouldn't be surprised to see 1/4" variation across the width of each
of those boards between the "wet" and "dry" states of the timber.

To fix the bracing, will zink plated wood screws will do,

If you can get good quality zinc plated yes but screws vary immensely
in the quality of the plating and in the steel. Brass or stainless
might be a better choice. I don't use anything but brass, stainless
or hot dipped clouts outside up here. Steel screws or nails just
doen't last.

As I like the look of wood, especially if it were stained to look more
like hardwood or a redder form of redwood, would something like a
coloured preservative be OK (such as tge Cuprinol stuff sold in B&Q for sheds and fences) ,

Be careful of shed/fence stuff, some is designed to work on rough
sawn timber and doesn't adhere to planed at all well.

or is regular oil-based paint (and lots of it) going to the greatest
longevity and the least need for repainting in years to come?

The full preservative, prime, undercoat, gloss system works very
well. We have some Dulux Weathershield (Ithink) that has been on a
south facing window frame for the best part of ten years. Where it
was thin on the beading it's come away but on the solid timber of the
frmaes it is still in good sound condition. Trouble is as bit's have
failed it all needs taking right back and redoing...

Take a look at (some of) the Sikens range, these weather but you just
lightly rub down and slap another coat on, not taking right back and
starting again.

--
Cheers
Dave.