When choosing paint for sandcarved items that will be placed outside, such as monuments, stone plaques, or bricks for walkways, you have to consider the exposure to all sorts of weather as well as being exposed to UV light on a much greater scale than any of the projects we have covered so far.
Regular spray paints, such as lacquers or enamels, can be used as long as they state that they are for outdoor use. I would, however, not overrate the longevity of these paints to your customer. Used on outdoor projects, I estimate the life expectancy of these paints to be six to 10 years, depending on where you live. Here in New Mexico at our altitude and high UV radiation, I estimate more on the short time than not.
For longevity, it is advisable to use paints created for the monument or sign industry. All of those paints are made with the knowledge that they are going to be exposed to the elements and need to last. One Shot is one brand of sign painter’s enamel that is widely used as well as Lithichrome paints or Stone Tone. These paints are available through monument or sign painter supply companies.