The oil and mining boom has burst in these Rocky Mountain highs, forcing J. T. Skinner to seek new markets overseas for drilling and tunneling equipment.

"The market here is so slow. . . very depressed at the moment. But there's activity in other parts of the world, especially in Africa and South America, and we're trying to get into those markets," said Skinner. His company, Card Corp. of Denver, currently exports only about 5 percent of its $6 million in annual sales.

He was one of about 300 persons who ignored the lightly falling snow to attend a conference here Tuesday for American business executives on how a new law allowing firms to unite as export trading companies can boost their sales overseas.

The conference is one of about 30 sponsored around the country by the Department of Commerce's International Trade Administration, and state and local organizations. It drew twice as many people as expected.

"I'm surprised so many people are here. But business is bad, and everyone is looking for new opportunities," commented Ray W. McDaniel of Samsonite Corp.'s international division. "They want to see if they can boost sales with some exports. But there are a lot of people here who never thought of exporting before."

The law allowing the formation of export trading companies was passed by Congress last fall and adopted as a jobs bill by President Reagan when the unemployment rate rose above 10 percent. It liberalizes antitrust regulations to allow American companies to join in export operations and, to assure the needed financing, permits banks to own export trading companies.

Japanese trading companies, credited with spreading that country's manufactured goods throughout the world, provided the impetus for the law, which Undersecretary of Commerce Lionel H. Olmer described last month as "perhaps the most important single piece of legislation on international trade to be enacted in a generation."

It came at a time when America's manufacturing trade deficit is reaching record levels--$42.7 billion last year and expected to go as high as $60 billion to $70 billion this year.

At Tuesday's conference, Deputy Assistant Secretary of Commerce Kenn S. George cited estimates by Chase Econometrics that exports generated by the law will increase the nation's gross national product by between $27 billion to $55 billion by 1985, add between 320,000 and 640,000 jobs to the economy and reduce the federal deficit by $11 billion to $22 billion.

Many trade experts consider those estimates overblown.

At the conference here, Roy W. Williams of Associated Pipe and Supply Co. said he was attending to learn how to expand his firm's sales of irrigation equipment to other countries in the Middle East and North Africa. He said the company already sells to Saudi Arabia and the United Arab Emirates and would like to crack new markets in countries such as Iraq, Syria, Egypt and the Sudan.

"We need help in making contacts that an export trading company may be able to supply," Williams said.

Skinner, of Card Corp., said he has tried using a Japanese trading corporation to increase his exports, but he hopes to get involved with an American concern.

"I don't know that much about it, though. That's why I'm here," he said.

Companies such as his--too small to be able to afford to run a major overseas sales effort themselves, yet dependent on exports to prosper--seem to be prime candidates for export trading companies.

In Colorado, for example, about 17 percent of the state's 4,050 manufacturing companies sell their products overseas, but their total export revenue is small--about $2 billion. "They are not selling a lot, but everyone is selling. The foreign buyers are waiting at the door," explained Donald L. Schilke, regional director of the International Trade Administration.

Many of the exports come from high-technology firms in what has come to be known as Silicon Mountain--a take-off on California's Silicon Valley. They are generally small-scale operations, and are strong in developing products, but not in marketing them. "That makes them a natural for a trading company," Schilke said.

There were some here, however, who sounded a note of caution. R. Ronald Guerriero, president of the Bank of Boston's World Trade Group, said there is "danger" that some small business official will think that export trading companies will provide an immediate answer to a company's economic woes.

"They don't yet," he said. "But they could in the future."