"To stain some surfaces" means to leave a mark or color on certain objects or materials, usually by accidentally spilling or rubbing something that causes discoloration. Full definition
Like his contemporaries Frankenthaler and Louis, Olitski stained the surface of the canvas in a technique that reacted against the gestural brushwork of Abstract Expressionism, and with its focus on material and surface, eliminated the illusion of depth. (rukajgallery.com)
Palazzolo has inflected these slightly stained surfaces with atmospheric modulations of hue, adding multiples of objects, for example floating light bulbs in «Tears of Things # 3» (2012) and flower petals in other paintings. (brooklynrail.org)
In 1969 I began a continuing series of stain paintings combining stained surfaces, with hard edged colored bands, painted in different sizes across the bottom (some of which had abstract writing in them) and later, colored bands of different lengths and widths on the edges. (ronnielandfield.com)