With most of America's overseas sales concentrated in big corporations -- 1 percent of U.S. companies are responsible for 80 percent of U.S. exports -- the Export-Import Bank of the United States has started reaching out to small businesses.

Ex-Im Bank officials, prodded by Congress, hope that making it easier for small and medium-sized American companies to export their products will help shrink the ballooning U.S. deficit in merchandise trade, which is expected to soar to a record $130 billion this year. See story, Page B1.

The bank, which is celebrating its 50th anniversary this year, helps American exporters by guaranteeing loans and providing credit at competitive rates to their overseas customers. Because of the big-business orientation of American exporters, however, critics in the early days of the Reagan administration portrayed the Ex-Im Bank as welfare for the rich.

The new small-business program is designed to change that perception.

"Many small and medium-sized businesses have the potential to expand their sales through exporting," said Ex-Im Bank President William H. Draper III.

"Some have been thwarted in their attempts to explore this exciting business opportunity by the complexity of the export process," he said. "Others are simply unaware of the resources and programs available to help them learn about exporting and evaluate whether it's appropriate for them."

To receive the first Ex-Im Bank certificates of achievement for exports, Draper singled out four small midwestern firms, all of which used the bank's credit-risk insurance and financing programs and its insurance agent, the private Foreign Credit Insurance Association. The companies are Vilter Manufacturing Corp. of Milwaukee, Crown Iron Works of Minneapolis, Advance International Corp. of Chicago and Lamina Inc. of Royal Oak, Mich.

The bank has targeted Illinois, Michigan, Minnesota and Wisconsin for tests of two programs to help small businesses increase export sales. One program offers incentives to banks and other lenders to make Ex-Im Bank-backed loans to exporters by accepting a broader range of collateral. The other will increase the availability of credit-risk protection to small businesses.

To press these new programs, the bank has set up a promotional arm. Robert J. Kaiser has been promoted from senior loan officer in the Asia Division to head the new Marketing Division.

"Our purpose is to explain what Ex-Im Bank is and the wide variety of services it can provide," Kaiser said.

Among other things, the Ex-Im Bank for the first time will guarantee bank loans to provide pre-export working capital to small businesses. To reach more such businesses, the bank will use the 117 regional offices of the Small Business Administration to distribute this working capital.

Because many commercial banks don't want to handle the extensive paper work needed to verify loans when the amount is small, the Ex-Im Bank agreed to let them skip some of the checks without being held liable for the loans when a problem occurs. And the bank will now guarantee loans for small businesses in service industries such as advertising, computer software, architectural work and engineering.