Willem de Kooning, who was born in Holland 100 years ago and died on Long Island in 1997, is one of the sturdier pillars of American art, along with John Singleton Copley, Frederic Church, Stuart Davis, Jackson Pollock and quite a few others.  The problem in dealing with him is, in some respects, similar to the one you run into with Pollock. De Kooning has been written about, mainly by Americans, in terms that might seem over-the-top for Rubens.  More tempests than Lear and Moby-Dick put together. Hysterical weather reports from some outer galactic fringe, accessible only to Hubble telescopes and art historians - such as John Mekert, who wrote the catalogue for the 1984 retrospective: "The creative force of eros has merged with the flux of a shapeless magma of light and unbound matter drifting towards congealment into form." Yikes!  The size of the reputation and the spread of De Kooning's influence rest on a rather small number of achieved paintings, not all of which justify some of the claims and comparisons that have been made for them.  The difficulty lies in testing the claims against the pictures because, with the colossal inflation of de Kooning's prices before and after his death, owners of his early works, which are the best, are getting tighter than a cross between Volpone and a giant clam when it comes to lending. (If Picasso's unimposing Rose Period Garcon à la Pipe could make $104m, what would de Kooning's canonical paintings of the late 1940s and early 1950s such as Woman I or Excavation fetch on the block, should the Museum of Modern Art or the Art Institute of Chicago decide to let them go? $70m? $100m?)  Even 20 years ago, when the next-to-last de Kooning retrospective was held at the Whitney Museum in New York, it verged on a fiasco for exactly that reason. Nobody could be persuaded to lend the best stuff; it was Hamlet without the prince - or Ophelia, or Claudius.  The owners cited problems of condition, which is a valid reason, but not the whole story. De Kooning's early technique - which, despite his orthodox Dutch academic training, entailed layering commercial house paint over implausible supports such as New York newsprint - was as much a recipe for long-term disaster as Picasso's newspaper collages, bound to darken and disintegrate beyond repair.  (De Kooning had supported himself as a house-painter in Hoboken, NJ in the years after he jumped ship to the US in 1926, but the experience seems to have taught him very little about the properties of house enamel.) The million-dollar babies, now today's $10m or $30m grannies, were falling to bits and there was no botox to adequately improve the ancient skins of these Loved Ones.  The subject of the fragility, indeed the evanescence, of so many "canonical" modernist images is a fascinating and depressing one, not yet fully broached. Restoring a Titian may be hell, but try restoring a Dubuffet collage of butterflies whose wings, alas poor Morpho, disintegrated into turquoise dust 20 years ago. Trip to the Costa Rican rainforest, anyone?  The strong possibility is that there will never be a full-scale De Kooning retrospective of really satisfactory quality. Few can afford to move the stuff, insurance costs being what they are, and nobody wants to risk damaging it.  It's hard so see how this situation could have improved since the 1984 show. Most of the important early De Koonings, including all the five Women, were missing from it. And the current offering at the Gagosian Gallery in New York, Willem de Kooning: a Centennial Exhibition, curated by David Whitney, shows further poignant signs of this drying-up of material.  The gallery spaces are filled, at a rough count, by some 40 paintings. Only five of these are from the late 1940s, and there is one good example of De Kooning's gritty-city evocations of close-up New York, Composition 1955, lent by the Guggenheim.  A terrific 1958 canvas, Suburb in Havana, with its lashing V-strokes of brown pigment on a beach-yellow, ultramarine-sky background, shows him at the summit of his form as a creator of landscape images, always a prime source of inspiration in De Kooning's work - abstract but not abstract, full of heat and a kind of demotic, people's-paradise energy.  And there is a lovely 1961 painting, one of his earlier responses to the landscape and seascape of Long Island, that table of luminous sea and sand which so resembled the flatness and rising, toppling cloud-forms of his native Holland.  Looking at this, as at the absent Women, it seems perverse if not actually impossible to think of De Kooning how he was once seen, as an abstract painter: he was as embedded in the experience of landscape and mutable light as the American "luminist" Martin Johnson Heade had been out there among the dunes a century before. Or is it simply that landscape is the ur-form of American abstraction, whether you like it or not; that the presence of landscape is so immense and powerful a factor in American experience that there is no point for a painter in trying to get around it?  The disproportionate bulk of the show, however, is given over to later, generally much later, paintings - and this is where the trouble starts. Anyone who has spent time on eastern Long Island and been immersed in its landscape will instantly recognise what De Kooning's landscapes of the later 1960s and 1970s proffer. You know those tidal ripples, that liquid light, those wobbly reflections of figures (probably clamdiggers) surrounded by the pink and pearly radiance of dusk on the flats. De Kooning is their poet laureate, no question of that. "There is something about being in touch with the sea that makes me feel good," he remarked to an interviewer in 1978, a few years before his death. "It is the source where most of my painting comes from." The sea, the mother.  But things get woozy and incoherent, as though mama were just a bit shickered on Amsterdam gin; brushmarks wiggle inconclusively and trail through the candied swamps of colour. One longs for the energy and definition that went into the earlier body-and-landscape images; they seldom appear, because De Kooning had fallen in love with a suggestive vagueness that carried intimations of aggression. The combination tended not to work, even though it suited the fascination with ambiguity and indeterminacy that his painting had always shown.  What rarely worked at all were his attempts at sculpture. Like gigantic nosepickings and twice as ugly, these are among the few things De Kooning made that don't succeed on any level at all, and although they preserve some of his linear energies they don't do so in an engaging way. Indeed, there has always been some doubt about just how much (to put it politely) De Kooning's aged and somewhat doddery hand had to do with their fabrication. Short of testing the original clay maquettes for identifiable fingerprints, which has not been done, there is no way of knowing for sure.  Fortunately, this centennial tribute does not extend to sculpture, and no such doubts, or none of that kind, beset the paintings with which De Kooning filled his last years. These have always been controversial: at one time, given that De Kooning by then was socially incapacitated by Alzheimer's and the brain damage brought on by many years of alcohol, there were rumours that studio assistants had been making them. But this is most unlikely, because any skilled faker would have done a better job of reproducing the old De Kooning "look". The new paintings looked nothing like the old ones and this was the best argument for their complete authenticity. Thin, ghostly, with spectral ribbons of dilute paint laid side by side, they are like the armatures of past De Koonings deprived of their flesh and texture. Sometimes they give you a sense of the deep energies that underlay De Kooning's earlier drawing, but their action is slowed right down. The paint that was once so oily and bodily is now more like watercolour. But the painter who did them was a painter still.  The artist Chuck Close, who lives in eastern Long Island, used to tell about going to visit De Kooning in his dotage: the frail, almost translucent old man with the thatch of Nordic hair, hunched motionless in that enormous studio by the sea. But, asked about the paintings that stood around, De Kooning seemed to come alive; he woke from his slump, stood straight up, and almost strode around the studio. The old brain was back at its old game of ruling, and the old painter had not died: he had simply faded away.  · Willem de Kooning: A Centennial Exhibition is at the Gagosian Gallery, New York, until June 19. Details: 001 212 741 1111.
Another De Kooning retrospective, another heap of woozy beach paintings. Will we ever see his luminous early works again, asks Robert Hughes.