TOH recommends "45% solids" when selectinig exterior house paint. is that 45% by weight or volume?
In a nut shell ....
The solids can account for as little as 25 percent of the gallon's overall volume to as much as half of the overall volume.
These solids are the binder and pigment ingredients that remain in the dry film that adheres to the substrate.
A higher solids paint will dry down to a thicker film, resulting in better hiding and protection of the substrate.
The rest of the product is the liquid carrier that makes it possible to apply these solids to the surface .....water in water based paints and petroleum distillate in oil base .
After application the liquid evaporates from the paint and you are left with the dry paint film.
Hope this helps. :)