It's a raw day on the Welland Canal, and the great lady of commerce is being primped and painted under wraps.

Heavy plastic blankets cover the huge steel gates on Lock 2. Underneath the covers, workers scrunch inside the 5-foot-wide compartments or bulkheads that honeycomb these gates. There are many pockets - the gates stand 80 feet high and 40 feet wide; each weighs 500 metric tons. The workers are painting where they first had sandblasted sections of each gate down to the metal.Air dryers and radiant heaters are huddled in with them, drying and warming so that the heavy protective paint will cling long and well. Freezing gusts of wind create weather bitter even for Canadians used to Great Lakes gales, ice and snow. Today, this western section of the St. Lawrence Seaway is one very cold and huge dry ditch punctuated by eight locks and 52 operating gates whose valves, cables and gears must function during the shipping season beneath 35 to 80 feet of water.

With the Seaway closed until March 31, an intense 11 to 12 weeks of winter work by some 750 workers is pressing on so that the goods of Duluth and Detroit, Chicago and Toronto, can be shipped to Europe and beyond for another year - the canal's 170th.

Workers are shoring up slipped banks, digging new trenches with lumbering excavation machines. They strip 2 feet of deteriorated facing off concrete lock walls that are 15 feet thick at the top and graduate to a massive 35 feet through at the foundations. New facing is laid on.

Some replace 1 1/4 -inch woven wire ropes that pull the great gates open and shut, in the age-old way not used in the 40-year-old Montreal-to-Lake Ontario section of the Seaway.

They tune up the motors and drums and huge round iron gears, the enormous chains and great concrete balance weights, which crank the ropes back and forth just as they did when installed in 1932.

''We are emphasizing electrical and mechanical work this winter - a lot of it operates under water and this is the only time we can get at it without divers,'' says Richard Corfe, vice president of engineering services for the St. Lawrence Seaway Management Corp. ''A lot of our electrical cabling goes back to 1932.

''It's a constant battle to keep ahead of it all,'' he adds, and the Welland takes up 70 percent of the annual 25 million Canadian dollars (US$17 million) that the Management Corp. allocates for maintenance of the Canadian part of the Seaway. Two of the five locks on the Montreal-Lake Ontario section are operated by the U.S. Saint Lawrence Seaway Development Corp.

''It's a constant battle to see that we give efficient transit today, and that we're still here providing efficient transit tomorrow,'' he says.

Not just tomorrow. Seaway planners have set a mandate of keeping the Welland Canal functioning pretty much as is until at least 2030.

''All of our engineering and our maintenance projects are geared toward at least that date,'' he says.

It is 170 years this year since the sailing schooner Ann and Jane opened navigation between Lake Ontario and Lake Erie, rising 326.5 feet to skirt the Niagara escarpment, through the first Welland Canal. Goods from the emerging industrial heartland of North America had their first clear run through to Montreal and the open Atlantic.

The Ann and Jane took two days to travel the 27 miles between the two Great Lakes on a canal of 40 wooden locks accommodating mostly sailing ships then. The canal was no more than 22 feet wide and had drafts up to eight feet.

That first canal was built to challenge the fierce competition to Canadian trade coming from the Erie Canal, built four years earlier in 1825. The Erie was relegating thriving Montreal almost to the status of a bucolic village.

This is the fourth Welland Canal incarnation, a key section of a great inland waterway that connects North America with itself and with the trade lanes of the world. It continues to be a work in progress, although no spring chicken itself. It opened in 1932.

Old as it is, today's Welland efficiently processes up to 19 ships a day through its eight locks, and could handle 25 or 26 if it had them. Through it sail ''Lakers,'' ships that are more like 700-foot-long self-propelled barges, and oceangoing ''Salties'' of similar size if not shape.

Last year, 3,492 Lakers and oceangoing ships carried 40.4 million metric tons of bulk and general cargo through the Welland Canal. Tonnage is down, but not because capabilities are waning on either the Welland or the Montreal-Lake Ontario sections of the St. Lawrence Seaway.

''We have moved 66 million tons in a year on the Welland Canal. We are underutilizing the system,'' Mr. Corfe says.

The Welland Canal comprises the ditch and seven lift locks that bring ships up 326.5 feet within a scant 7.2 miles - between Lake Ontario and the top of the Niagara escarpment. The channel then runs 17.3 miles through flatlands to the eighth, shallow-lift, control lock at Lake Erie. Piers projecting 2.5 miles into the two lakes account for the rest of the 27 miles of canal. Standards are the same on both sections of the Seaway: 27 feet minimum of channel depth, 30 feet at the sills; locks 766 feet long and 80 feet wide; average lift per lock of 46.5 feet.

Welland Canal locks 4, 5 and 6 are the well-known flight locks - ''three locks joined together, rising about 135 feet within a length of 2,400 feet or three ship lengths,'' Mr. Corfe explains.

Each is a double lock, with two sets of gates serving two channels. For this section, the Welland Canal is a water boulevard. Ships coming into the middle of the continent pass ships heading out. In effect, you have six locks in three. Depths are 80 feet.

''This part of the Welland is unique,'' Mr. Corfe says. ''There's nothing like it anywhere else in the world.'' With the importance of U.S. and Canadian inter-Lakes and international shipping, it may be long past 2030 before the Welland Canal is replaced, despite the ever-increasing size of ships.

''You couldn't shut the traffic down for the time it would need to rebuild it, so you'd have to build another canal in parallel,'' he says. ''We have had for 30 years the land for a parallel system, another canal.''

But the billions of dollars that would be required and the environmental concerns that would be raised seem too daunting. ''So we are switching within the corporation to thinking we may never actually replace it but just go on maintaining and refurbishing it,'' Mr. Corfe says.