His first artistic production was influenced by Surrealism. Following were the American abstract expressionism and informal French to leave visible traces in his works, which from 1950 onwards were not only more pictorial. Deeply suspicious of rationality, he investigated the potential of dreams, madness and the subconscious; to these ends he co-founded the Hundsgruppe under the influence of French Surrealism in 1950. Austrian painter, printmaker and photographer, he had almost no academic training as an artist. From 1948 to 1951 he produced Surrealistic drawings representing underwater scenes and mystical forms, rendering these fantastic images in pencil as a densely worked surface.
From 1951 to 1954 he worked on a series entitled Blind Drawings, in which he studied optical disintegration and the destruction of form, replacing pictorial composition and illusion with the immediacy of accidentally encountered textures.
From 1953 to 1965 he devoted himself principally to a series of Overpaintings, in which he obliterated his early expressive drawings or pictures by friends with whose work he was in sympathy, to produce almost monochrome paintings dominated by black or red.
From 1956 Rainer became concerned with religious theories and practices, particularly in a group of paintings dominated by cruciform shapes. The interest in extreme emotional states hinted at in such works became even more pronounced in 1963, when he began to collect paintings by the insane, and in 1964, when he experimented with hallucinogenic drugs.
Other cycles of works ranging from explorations of the body language, especially through photography on which intervenes graphically (Face-farces, 1968-75; Bodyposes, 1971-76), to the Sovrapitture of funerary masks (since 1977), the cycle Hiroshima (1982), to the series Giotto u.a. (1999).
Since 1994 he has also experimented new techniques and materials (sheet ribbed aluminum, cartons milled with geological structures, astral, etc.) in the series Mikrokosmos-Makrokosmos, where has renewed his paintings with his fingers, and in the elaborate series Schleierbild (1997-98) and Traumland (1998-2000).
Among his numerous solo exhibitions we have to remind, in particular, the exhibition organized by the Sao Paulo Biennale in 1996, the solo exhibition on his engravings in the Kunstmuseum in Bonn in 1997, the retrospective in the Stedelijk Museum, Amsterdam (2000) and in the Art Gallery modern Bologna (2001).