@Vermis:
Er, I didn't say that the article explained why it was harder on brushes? All I said was that it explained why metallic paint is different to normal paint and that it was interesting.
Since you seem to want a more specific explanation, try this:
The hair in natural bristle brushes has this structure:
Normally, water-based model paint consists of finely ground pigment suspended in an acrylic medium. Some paints (like P3) have a liquid pigment in an acrylic medium.
Now, during normal use, some of the pigment gets trapped under the cuticle scales of the brush; if the brush is not regularly rinsed, the buildup is forced under the cuticle scales which makes it harder to remove than by rinsing alone. A special brush soap or a brush restorer can help to work the pigment from the brush in these cases.
If the paint buildup starts to dry in the brush, then the damage to the hair is greater as the removal is harsher.
When a brush gets old, the hairs may break or go 'limp' because they have been damaged partway down their length. The hairs also are no longer smooth as the lifted scales roughen the surface. This can cause the brush to lose its point as the hairs will no longer lie together properly.
Also, the bristles in brushes are tightly gripped and glued to the brush. When paint buildup gets into cuticle scale near this anchor point (protected by the ferrule), it becomes almost impossible to properly remove and causes permanent damage to the brush as the buildup worsens through repetition. This also causes fishtailing or splaying, and can damage the hairs near their base, causing them to break (i.e., your brush will lose hairs). This is partly why it is recommended not to dip the brush in paint up to or past the ferrule.
Because metallic paints contain mica fragments (mica platelets), and these platelets have 'sharp' edges, their buildup under the cuticle scales of the brush hairs can be much more damaging than normal.
Furthermore, some metallic paints contain other additives (like Isopropyl Alcohol) to thin them and improve their application. These additives can dry out the oils found naturally in hair much faster than regular rinsing.
To help keep brushes in good condition and restore these oils to the hair, many folks recommend a good leave-in hair conditioner is applied to the brush when you have finished your painting session; a good rinse of the brush in clean water before you start again is all that's needed afterwards.
Hopefully that explains it a bit!
