|there are black streak marks showing up on many
| of the boards.
| Any idea what these might be and more importantly how I "might" be
| able to get rid of them???
|

Probably mildew. A strong bleach solution will make
it fade out. Something like 1 or 2 parts bleach to 5
parts water should be about right. The traditional
recommendation is bleach and TSP, but the TSP is
only for cleaning. It won't affect mildew.

After you do that you might want to consider some
kind of finish. Just because it's PT doesn't mean it
doesn't need a finish. The first reason would be
simply because PT is very ugly. The second is to
protect the wood. Like cedar, it will resist water
well but it doesn't last as well against drying from
sun exposure.

Your options are somewhat limited. It's nearly
impossible these days to get a solid, oil-base deck
stain. Water base deck stain is junk. It just wears
off in a year or less. It also shows dirt badly. Semi
transparent oil stain won't look as good over PT
because the wood is so ugly and dark in the first
place, but it's a pretty good choice. Two or 3
coats of something like cedar tone should almost
cover, and is easy to re-apply.

Sherwin Williams has come out with an acrylic product
called Super Deck that's supposed to put a very tough
finish down, and even fill cracks up to 1/4". It sounds
interesting but farfetched to me, but I'm curious. The
problem with something like that, though, is that it's
unproven, and if it turns out to be a lemon you've ruined
your deck.

Another thing I've used when I've wanted a nicer looking
paint finish with a rough PT deck is to mix Benj Moore
high gloss oil Impervo with liberal amounts of boiled
linseed oil and thinner, so that it's a thin, stain concsistency.
The result is a glossy stain that provides solid color but
doesn't peel, and it holds up pretty well. (The drier the
wood, the better it will work.)

I use a similar mix in black for rusty wrought iron railings.
The thin blend soaks in nicely and doesn't peel. One caveat:
I think they've reformulated HG Impervo since last time I
used it. I haven't tried the new version. But the general idea
is to just get the best quality gloss, exterior oil paint you
can and use that as basically the drier and tint for the
linseed oil concoction.