Micronized Zinc Oxide

Very small particles (especially nano particles) are far more reactive than their macro counterparts (this is a function of their far greater surface area). Any of the defects found in zinc white would be exacerbated in the micronized version and magnified in a nano form.

​Nanoparticle zinc oxide is the grade typically used in sunscreen lotions and other applications. Typically, micronized particles are between 0.1 micron and 100 microns, however the grinding process may result in nanoparticles called ‘fines’ which are smaller and in the nanoparticle range. Nanoparticles are usually sized between 100 and 2500 nanometers (100 nanometers is 0.1 microns). The grade of zinc oxide pigment used today in paint is micronized produced by the American or French process. Nanoparicle zinc oxide is even more reactive in varnishes and oil paint than micronized zinc oxide, as Brian wrote.

Although zinc oxide absorbs UV light, there are better substitutes in varnishes, such as hindered amine light stabilizers (HALs). There are also better matting agents, such as fumed silica for use in varnishes.

Thanks George, you point out a relevant fact. Micronized and nano are not the same thing. The funny thing is that they are often used interchangeably in conservation literature. There are many papers on nano particles used in conservation where the particle size is really not nano at all. It is important to be precise here and I have edited my comment to reflect this.