Once you squeeze it out of the bottle, acrylic paint goes through two different processes: drying and curing.
We all know how this works: water evaporates from the paint, leaving behind a solid, durable coating. With some paints, like watercolor, gouache, and the elementary school version of tempera, that's the end of the story. They're solid simply because they're dry. Get them wet, and they become workable again. But that doesn't work with acrylics, oils or enamels. Why not? Because these paints don't just dry, they cure too.
When acrylic paint hits the air, a chemical reaction begins. Molecules in the paint bond to molecules in the air, forming a tough, durable film. Some solvents may be able to break down this film. But once the reaction occurs the paint will never be workable again.
Although you can usually tell when paint is dry, curing is a little more difficult. The process can take hours, days, or even weeks to reach its conclusion. And all the while, the paint gets harder and more durable. That's why fresh paint can be very fragile, and why it's often more difficult to strip old paint from models and brushes.
In 2-dimensional painting, this pops up a lot when I want to use technical pens or crow quills on top of acrylic. If the acrylic isn't completely cured, the pen nibs can dig in and catch. That gives me wobbly lines and can even tear or strip the paint. If you use tecnhical pens to draw black lines on your models, you may have encountered the same thing.