Why Does Acrylic Paint Chip?
As I finished writing my article on painting Deadzone terrain I mentioned that it was important to wait 24 hours before adding a clear-coat to a model. This isn’t something I see discussed often within the hobby and I thought it would be a great opportunity to try to understand our chosen medium better.
When I desperately want to paint but can’t I try to cope with my frustration in a number of ways. Sometimes it is shopping for stuff I don’t need but usually it is reading anything I can find online even tangentially related to my hobby. I especially love painting tutorials and how-tos. It’s an obsession of mine and it has lead to a point where I’m familiar with a wide range of painting techniques but not proficient in any. It feels like I never paint the same way twice.
Acrylics aren’t just the undisputed king of hobby paints, they are also a medium used in the fine arts community and I’ll happily read and watch tutorials of people painting landscapes and portraits. Through this I have learnt a number of interesting things about acrylic paints that have been incredibly useful for my hobby. The most basic is how acrylic paints dry.
The Chemistry of Acrylic Paint
Acrylic paint is made of three substances. Pigment, water and acrylic polymer. The role of the pigment is obvious. The water acts as a solvent to keep the acrylic polymer molecules from touching and forming a solid mass. The acrylic polymer, once the water has evaporated, creates the paint film which binds the pigment to the painted surface. This is a simplification, of course, there is much more chemistry to Acrylic paint than what I just covered. In the future I’ll cover more of the chemistry when I talk about how to properly thin acrylic paint.
Once enough of the water evaporates of the surface of the paint it becomes dry to the touch. This is what most people think of when they talk about the paint being dry. Certainly additional coats of paint can be added at this time without worry about disturbing the paint underneath. At this stage the acrylic polymers have bound enough that they cannot be put back into solution by water. The drying process isn’t complete, however, as there is still a small amount of water trapped in the paint. As the polymers start to bind together capillary action forces out the remaining water until finally the entire paint layer has cured.
Once the paint has cured the film will have reached it’s final strength. Cured paint will be less likely to chip and rub off. There is also no more water evaporating from the paint and a clear-coat can be applied. Any sooner and you risk clouding your varnish due to the excess moisture.
Fortunately for us hobbyists we use extremely thin layers of paint. It is estimated that a 1/4 inch thick layer of paint can take years to fully cure. It takes me long enough to paint an army without having to wait years to varnish and start playing with them.
How about you? Have you had a paint job ruined by chipping or a cloudy varnish? Leave your horror stories in the comments below. You can also find me on Google+ and Twitter where a simple “Hello” would make my day. Don’t forget to subscribe for more painting tips.