Nanite Clouds

Everyone who enjoys art hates an art critic. Especially those anti-social critics that express their views with spray cans of paint, buckets of filth or use squeeze bottles filled with destructive agents. 
Before in order to protect art, the work was hidden behind barriers of glass or plastic to prevent these critics and vandals from defacing the artwork. However, doing this caused many complaints about difficulty when viewing or photographing. With those most famous artworks made worse by the crowds who wanted to see as well.
The Draco Foundation generously gave to a research corporation the funding to create a cloud of nanites that would shield artwork from acts of vandalism and damage. 
Nanites are microscopic machines that are programmed to perform a variety of tasks. In this regard the aforementioned nanite cloud was devised to act as a shield. Applied as a microscopic layer on the artwork, the nanites would collect any sprayed or tossed chemical or matter preventing it from collecting on and damaging the artwork. 
In this way unwanted paint, bodily fluids, pollutants and chemicals which would damage the artwork would be prevented from touching the work of art. 
This particular nanite cloud was first tested at a derelict site by observing if the nanites could prevent paintballs from splattering on the sides of buildings. Stone, concrete, wood and glass were all tested. It was found that regardless of how heavy the offending material was applied that the nanites would prevent said material from marring the art.
Where the nanites failed to protect was if a direct attack was being made upon the art. Knives, box openers and the like were too much for the little nanite machines to deal with. Fortunately with security screening at museum entrances as well as the good work of the Art Preservation Agents, a very good layer of protection has been afforded to the protection of the art pieces.