San Francisco's graceful guardian; The mammoth, 48-year-old span is being refurbished - at night, huge slices of its six lanes of roadway are cut away and replaced; by day, 'platform flying' painters are recoating every nook and cranny

A tiny, two-man (three-if-you're-thin) elevator ascends so slowly a visitor thinks it's standing still in the darkness of the Golden Gate Bridge's north tower. Vertical progress is marked in the dank air by the lengthened echoes of bridge workers and the passing of steel cells knitted together with fist-sized bolts in a honeycomb of structural support.

At the top, 700 feet and many minutes from the bottom, the hatch from the hollow of the highest tower strut cracks open and a flood of daylight and cool salt air makes it obvious the elevator has not been standing still. The ascent has gone far above the sound of traffic whizzing by directly underneath, higher even than where birds roost.

The first-time gawker takes the wire railing in a white-knuckle grip, looking between two feet at pavement and sea below, and then out at a 360-degree panorama of sea, bay, city, and countryside. But the five painters hardly notice where they are. Within minutes, painters Martin Gonzalez and Bud Wiley have been sent over the edge of the strut on a 12-foot plank, a paint rig suspended by two 5/16th-inch wires.

Their job is called ''flying,'' because sometimes that's exactly what they do when high winds catch the platform, blowing them as much as 40 feet away from the bridge. Despite the height and wind, they're supposed to wrestle 100 pounds of sandblasting force over every inch of steel on the art-deco styling of the north tower and then paint it.

They've been working near the top for four years, in part of a project to repaint the whole bridge. The project began in 1965. And that's part of the reason they're here - the job security that goes with the bridge's permanence. One of the modern wonders of the world, this bridge was designed to last another 150 years, and at that rate, Mr. Gonzalez and his fellow workers know they'll have permanent jobs.

''Sure, there's always scenery, there're ships coming in and out,'' Gonzalez says. But aesthetics places second to pay ($40,000-plus a year) and job security. ''I'll probably work here the rest of my life,'' he adds.

Dan Mohn wouldn't mind working here the rest of his life, either.

''After this, no other bridge in the world would interest me,'' says the bridge's chief engineer.

He likes any excuse to get out on the bridge and leave his office, where coveralls, hard hat, and climbing boots lay ready in a closet behind design drawings of the bridge.

In fact, during 80-mile-per-hour winds on Dec. 3, when the bridge was closed for only the third time in its 46-year history, Mr. Mohn was the lone figure out on the main span. This is the fun he waits for - to see the bridge ''work.''

''I wanted to see it performing, so I was too excited to be afraid,'' Mohn grins, describing his venture onto the bridge when it was closed in a worse storm a year ago.

Photos show him in a lone vehicle on the bridge, where a normally straight center white line was arced eastward, the center span blown eight feet sideways.

''There were five-foot vertical oscillations once every 15 seconds. . . . The expansion joints (visible on the bridge roadway), finger-like joints, were moving in and out two feet every four seconds,'' says Mr. Mohn, describing what no engineer's blueprint could show.

The 1.2-mile suspended section of the bridge, he explains, was designed to lift and move almost like a mechanical device with moving parts. Though bridge movements are very noticeable - and disconcerting - to the average visitor, Mr. Mohn is quite certain about its permanence.

''The safest place I can think of in an earthquake is the middle of the Golden Gate Bridge,'' he says confidently.

No matter how you look at it - through the lenses of tourists' instamatics, through commuters' windshields, on an engineer's drawing board, or just through the binoculars of San Franciscans in their living rooms - everyone sees the Golden Gate Bridge as a venerable novelty on the order of the Grand Canyon.

Its rank alongside centuries-old natural wonders of the world is a testament to the dream of the bridge itself as well as the engineering genius that holds it up.

But unlike wonders formed by nature, the bridge was built in defiance of nature and must be guarded against the very elements that form natural wonders. Constant maintenance, painting, and road work keep the bridge looking and running as it did at completion - a festive orange banner, grinning in the face of the odds against its success.

Engineers in October launched the biggest fix-it project here since the bridge was built. A complete replacement of the six-lane roadway and sidewalk - which, except for three storms, have never been closed to foot and auto traffic - will take two years and cost $61 million.

The bridge itself was built in four years at a cost of $35 million. It was completed in 1937. Considered a fantasy at the time, the bridge was originally financed by a private bond issue. But, ironically, new construction on the bridge, which is the only route into the city for thousands of commuters, will be financed today largely by federal highway funds.

Other major projects have included the replacement of all 200 pairs of vertical wire ropes (nearly three inches in diameter) and the current painting project - 82 percent complete - using a vinyl paint expected to have a longer life (25 years), says Ross Salazar, painting superintendent. More than half that time has passed since the first of the new paint was applied in 1965.

But the new roadway replacement project, bridge officials say, is the biggest and toughest yet.

Mindful of commuter needs and the importance of bridge aesthetics - especially for the big tourist industry - the bridge district management stipulated that the bridge would not be closed during the project.

All work is being done between 8 p.m. and 5:30 a.m., and only four of the bridge's six lanes are being shut off. Crews haul out their equipment and tools - and put them back - every night, explains Charles Madewell, project manager for Dillingham Tokola.

Their piecemeal work involves sawing out a 50-foot by 30-foot section of concrete roadway and sidewalk each night. The same night, crews must replace the concrete with two preformed road sections to be laid down on the exposed floor beams of the bridge structure.

At the same time, the crews will be replacing and narrowing sidewalks, giving an extra two feet to the 60-foot wide road. The new ''orthotropic'' surface is 40 percent lighter than the old concrete, and the steel supports of the new structure will form the roadway itself.

More than 750 20-ton highway panels and 325 sidewalk panels will be individually designed for the particular spot each will fit into.

Detailing of panel components is done in Birmingham, Ala., fabrication of them is done in Salt Lake City, and assembly of the components is done in Napa, Calif., Mr. Madewell explains.

''It has to be done carefully. The consequences of parts not fitting together right are pretty serious,'' says Dan Mohn. But he adds that it's a good thing the construction is being done now, while there is the time to do it carefully. Over the years, corrosive salt had crept into the steel-reinforced concrete, saturating each cubic yard with three pounds of salt, leaving the roadway with only 10 years of life. The new deck should last the lifetime of the bridge, planners say.

Despite the need for replacement of the deck, Mr. Madewell says he's impressed with the bridge's timelessness. ''It's absolutely worth preserving.

After all, the only thing we're doing is replacing the roadway'' after all these years, he says.

While considerable attention and money is being poured into the structure of the bridge, other complexities have grown up around it. The fiefdom of the 19 -member Golden Gate Bridge and Highway District, a state-chartered and independent commission representing the five counties that originally threw support behind the building of the bridge, has come to include a whole transit system for the suburbs north of here.

Having opened the way to the suburban development of Marin County and its northern neighbors, the bridge management fell heir to the subsequent responsibilities of managing the growing daily influx of commuters the bridge had created from the north. (An average of 50,000 vehicles a day use the bridge.)

At one flash point two years ago, a powerful California politician, upset because he got stuck in a bridge traffic jam, set the legislative ball rolling to take bridge management out of local hands and turn it over to the state. The plan didn't survive, and once again the bridge withstood the elements.