With its 19 Winslow Homers and eight John Singer Sargents, "American Traditions in Watercolor: The Worcester Art Museum Collection," which opens the fall season at the National Museum of American Art, is a rich and lovely show. It is exasperating, too. The artists represented -- they all paint with colored water -- are so good it isn't fair.

They make it look too easy. If you have ever tried to do it, you know that watercolor painting is just about impossible. Everything goes wrong.

The paper is too wet or dry, your brushes are the same, the water used to cleanse them refuses to stay clean. The colors you apply won't stay where you put them, they run this way and that. And if you try to paint outside (as did most of these artists), things go from bad to worse. Judith C. Walsh, who works as a conservator (but must also be a painter), begins her essay in the catalogue with an admirable list of the medium's indignations:

"The paper buckles when wet; the colors puddle and run into each other; fine washes laid on top of each other marry and become muddy; mistakes can't be painted out; underdrawing shows through the washes of color. Working outdoors made the task yet more arduous -- especially when the watercolor manuals would nag, nag, nag the artist to create paintings that appeared spontaneous. How did artists ever learn . . .?"

Through errors and through trials. Tons of crumpled paper, and countless mumbled curses, lie beyond the disciplined, exquisite pictures in this show.

The first watercolors produced in America were painted by the Frenchman Jacques Le Moyne de Morgues in northern Florida and Georgia in 1564. The first manual on the medium, Archibald Robertson's "Elements of the Graphic Arts," appeared in Philadelphia in 1802. Dry cakes or "pans" of color have been manufactured in this country since the early 1820s. Once upon a time, watercolor sketches were nearly as common as are snapshots now. Watercolor sets are still sold in the dime stores. It's a democratic medium. Amateurs, however, are missing from this show.

Edward Hopper, Maurice Prendergast, Charles Sheeler, Andrew Wyeth, Charles Burchfield and Rockwell Kent are among the artists showing. Other admirable painters, with less familiar names but comparable skills -- Henry Bacon (1839-1912), Frank W. Benson (1862-1951), Ogden M. Pleissner (1905-1983), Earl Horter (1881-1940) and New Hampshire's DeWitt Hardy (b. 1940) -- are also represented. They're professionals, the lot of them. But do not feel excluded if you're a mere beginner. If you've ever tried to paint, you ought to see this show.

It teaches as it pleases. Because watercolor painters must accommodate their accidents and live with their mistakes, their pictures carry with them a sort of visual pedigree. If you look at them intently, you can see how they were made.

The white fangs of the reptile seen at lower left in Sargent's wondrous "Muddy Alligators" (1917) weren't painted, they were scratched. Their whiteness is the whiteness of the paper. The blue-gray belly of a nearby beast, Judith Walsh observes, "has a unique fibrous pattern, which Sargent created by quickly tamping the wet paint with cotton. In raking light, stray hairs of cotton wool can be seen embedded in the paint." Sargent had a thousand tricks. To achieve the look of sunlight passing through the trees above the submerged boat in the painting he called "Derelicts" (1917), he used a wax "resist," rubbing at the paper with a colorless wax crayon, or perhaps a sharpened candle, so that his paints would be repelled when he put his washes down.

Homer was as cunning. He used the finest tools. A box of his dry paints, and two of his brushes, are among the works on view.

"The two Winsor & Newton watercolor brushes that Homer used," writes Walsh, "are still remarkable for their fullness, flexibility and beauty. The brush hairs of 'finest brown sable' were rolled into place by hand, their ends wrapped in magenta silk to cushion the hairs, then 'tied with gold wire' before being set in the natural quill handles." He used cotton, too, and blotting paper, porcelain-tipped burnishing tools, and scrapers. He used water most of all.

Walsh describes, in detail, the techniques Homer employed in painting two superb pictures here on view.

"Crab Fishing," the first, is a relatively early work of 1883. "He first drew the boat and figures in graphite, carefully delineating such details as the girls' features and wisps of hair blowing in the wind. He then colored the primary forms with dense washes." Apparently displeased with the line that describes the dinghy's starboard gunwale, he scraped away the paint and then reblotted in the sky. He also scraped away the boat to delineate the oar and crab trap. To suggest the dripping water falling from the trap, the small smooth wave beneath it was mottled with a sponge.

Homer's "Fishing Boats, Key West" is even more amazing. It was made 21 years later. Here it seems that water, the palest of washes applied wet-into-wet, does most of the work. He'd drop water on the washes to indicate the threatening texture of the sky. "First he applied two blue washes, one brilliant blue and the other gray-blue" to represent the sea and the shadows of the boats. "Before they had dried completely, he introduced a flood of clear water from the bottom left corner. He then rocked the sheet to manipulate the direction and spread of the water . . The flood retreated, and as it dried it deposited the carried pigments at its farthest reach. At this point, Homer daubed on more water in dots to indicate the first drops of rain in the storm. Maintaining perfect control, he magnificently produced the fleeting natural effects of wind and rain on water."

For many of these artists, water is not only the medium that they paint with. Its foamings and its drips, its waves and its reflections, are major subjects of their art.

Of the 75 pictures here, 46 depict water. The oldest painting in the show, by Edward Savage (1761-1817), is "Falls of the Passaic at Patterson," dated 1806. Waterfalls and cresting waves, swamps, limpid pools and duck ponds, are seen throughout the show.

Sargent is terrific, but nobody paints water as well as Winslow Homer. In his "Saguenay River, Lower Rapids" (1897), the water is so wild, its shadows are so black, its frothy foam so yellow, and its curving rush so heavy, one can almost feel its weight.

Edward Hopper's water doesn't rush, it sort of stands there. The wave that rocks his sailboat in "Yawl Riding a Swell" (1935) looks as if it might have been carved out of blue stone. The boat's sails are as solid. They look like marble in bright sun.

The good Andrew Wyeth in this show, "The Rope" (1957), (though it also shows a boat) is the least watery of these watercolors. Its boat is not at sea, but stashed within a barn. And the brush the artist used to depict the tangled rope above it has been squeezed almost dry.

Watercolor is just the thing for atmospheric subtleties, for clouds and mists and stormy skies. Perhaps that is because the medium demands speed, perhaps it is because the white light from the paper comes shining through the colors like sunlight through fog. Skies produced in oil, and painted in the studio, often have a look that in comparison seems canned.

The Sheeler on display, of factories and freight yards, has a dustiness that's rare here (and a linear precision that is uncommon, too). The breeze that bends the trees and tosses the yellow flowers in Charles Burchfield's "June Wind" carries through the galleries. It carries the white cloud (done with a single brush stroke) that moves across the Alps in the Arthur B. Davies landscape of 1928. It tickles the cold water of Frank Benson's half-oriental scene, "Eider Ducks in Winter" (1913). It ruffles the ladies' dresses in Prendergast's "Low Tide, Beachmont" (1902-04). It suggests moisture, ease and freedom. It rushes through this show.

The Worcester Art Museum, founded in 1896, has been collecting fine watercolors since the beginning of this century. It acquired its first Homer, "Old Friends" (a scene depicting a woodsman and a tree), in 1908. Between 1910 and 1912 an additional 75 were shipped to the museum for the director's consideration. Twelve of those he picked are included in this show.

They were relatively dear. Their prices ranged from $540 to $1,000. "Kindly remember," complained the director, "that Worcester is not closely in touch with the art world and that I have great difficulty in making people understand why 'such things' should bring 'such prices.' "

The Sargents were less costly. In 1913, at a time when the museum was willing to spend $25,000 for a single Sargent portrait (done, of course, in oils), it acquired a set of 11 of his watercolors for $2,750.

Though the museum owns a Sol LeWitt of 1982, and a wordy William T. Wiley of 1981, its collection, on the whole, is comparatively conservative. It owns nothing by John Marin, Arthur Dove or Charles Demuth, those three masters of the medium. Still its collection of American watercolors is, by any measure, grand.

With its John La Farge retrospective on display upstairs, the National Museum is now providing more lessons in the techniques and possibilities of watercolor painting than any studious painter could possibly absorb. Both shows reward close scrutiny. A $100,000 grant from the Digital Equipment Corp. helped pay for the Worcester exhibition's tour. The show closes in Washington Nov. 2.