Norminn wrote:
Ether Jones wrote:
Norminn wrote:

Quality paint rarely needs thinning. To thin it too much is to weaken
it's ability to stay on securely.


The question on the table is, *why*. See further discussion below:

EtherJones wrote:

For example, say I pour out two equal portions of unthinned latex into
separate containers. I paint one board with 2 coats using the first
portion (using it all up). I thin the second portion 50/50 with
distilled water, and paint the second board with that, applying coats
(and letting them dry) until the second portion is used up. Both
boards now have exactly the same amount of pigment and binder on them.
Will the paint on the second board fail because I thinned it? _Why_?
(It's the "why" part I am seeking to understand).


Norminn wrote:

Well, try this: take four boards, each 12" wide by four feet long. Cut
them in one foot lengths. Lay the 16 pieces of cut boards in four rows
of four. Should cover 16 square feet. Now pick them up, throw away 8
of them. Use the remaining 8 pieces to cover the same area.


Your example is flawed; it is in no way analogous to what I wrote.
Please reread my example more carefully. In my example, there was no
binder "thrown away". Both boards have exactly the same amount of
binder (and pigment) on them. So what makes one "weaker" than the
other? Are you claiming that the added water somehow prevents the
binder from polymerizing properly? And if that is what you are
claiming, where did you learn this? Could you please cite some
technical references.


I thought my example was pretty good, to illustrate that the diluted
binder has fewer molecules to hang together and cover your wood.

The thinned latex has exactly the same total number of molecules of
binder as the unthinned latex. Read the example again. Adding water
does not reduce the number of binder molecules, it just increases the
number of molecules of water.

Because the second portion has additional water, it is thinner and
therefore more coats will need to be applied to use it all up. But
once it's all used up, you've applied the same total number of binder
molecules to the second board as you did to the first.

If you want a treatise on paint chemistry, go find one.

I've been looking. Haven't found one yet.

You obviously understand factors that made your paint job fail,
so why pursue methods that aren't recommended?

Because I want to understand WHY they aren't recommended. I like to
understand why I'm doing what I'm doing. If the label on the paint can
says "Do not thin", I want to know why. By knowing "why", I can
determine under what circumstances it might actually be permissible,
even beneficial, to thin (even though the label says "no").


Common sense serves better than intimate knowledge
of the chemical compounds, it seems.

Common sense is good yes. But sometimes what passes for common sense
is a collection of urban legends and anecdotal experiences. That's why
it's good to ask "why". I'm not saying that's true in your case. You
seem to have some substantial experience.