George E. Cawthon wrote:

Thinning aggressively is a bad practice, you may
get the color to move into cracks but you end up
with mostly pigment and very little base to hold
the pigment to the wood.

Thanks for your lengthy response George. Some of your comments puzzle
me so I'd like to ask a few questions.

I don't understand your statement that I "end up with mostly pigment
and very little base to hold the pigment to the wood". If I thin
50/50, doesn't the ratio of base to pigment stay the same? In other
words, what happens to all the base? Isn't it still there, just like
the pigment is? Or is there some sort of chemical reaction where the
water destroys the base? I realize that by thinning, I don't get as
thick a coat as I would otherwise, but I figure that "something" is
better than "nothing", since the unthinned paint simply does not get
down into the fine cracks in the wood.

The cracks I am talking about are not large cracks that could be
caulked. I'm talking about many fine splits in the wood; of the order
of the thickness of a piece of paper. The unthinned paint simply
cannot get in there thoroughly, no matter how aggressively I brush it.
And, as it cures it leaves pinholes where the splits are; pinholes
where water could get in. If I thin the paint 50/50, it's still
fairly thick, but it is able to soak down into the splits instead of
just bridging over them. Then when I let it dry and paint over it with
unthinned paint, I get a continuous coating with no pinholes.

I tried scraping and washing a portion of the porch floor, then letting
it dry thoroughly and painting it with 50/50 thinned latex. You can
see the paint soak in to the fine splits, and when it dries it
absolutely doesn't rub off - it is very tenecious. I intend to cover
this first thinned coat with 2 additional coats using unthinned latex.
I'm not trying to go cheap on paint; I'm trying to get the wood coated.
Also, there are nooks and crannies in the porch where it's very
difficult to get the unthinned paint to go; a prime example is between
the deck boards. If I use unthinned paint, it wants to "bridge over"
adjacent boards instead of soaking down between them. Then, when you
walk on the boards, the slight relative motion between adjacent boards
causes the paint "bridges" to fail and expose bare wood. If I paint
these areas with thinned paint, it penetrates between the boards and
coats the hidden edges (where water drips down through). In this case,
when putting the unthinned overcoats in these areas, I would be
carefull to brush out any paint bridges.

If you look at most
latex primers it says to not thin or to use a
maximum of 10 percent thinning fluid

The label says "do not thin" but it does not say *why*. Since I
intend to thin only the first coat, and then go over it twice with
unthinned paint, I would like to understand if (and why) thinning is
still objectionable.

Or, preferably, use sound board with no cracks.

I understand that this would be optimal. In another universe I'd love
to do that. But I don't have the time or money to rip up my porch
floor. I'm constrained to work with what I have, with the wood in
place. There is no rot, but in many areas the wood is weathered, and
the paint is blistering and peeling (actually, the latex topcoat is
adhering to the primer, but the oil primer is peeling from the wood).
It's because of my unsatisfactory experience with oil primer that I am
exploring other approaches.

the surface coated with a high quality porch paint.

In the painful process of learning, I've had the "pleasure" of using
several different paints, some of them awful. The one I am using now,
and with which I am fairly impressed, is "Best Look Premium 100%
Acrylic Latex Satin Porch & Floor Enamel" from the local hardware/paint
store. It says on the fine print on the label that it's made by
Sherwin Williams. It costs about 25 bucks a gallon. It is very thick
and creamy, easy to apply, and coats very well.