In its first such action against a Japanese-made automobile, the Federal Trade Commission announced yesterday that owners of 1975-78 Honda cars may be entitled to new fenders or cash reimbursement because of a premature rusting problem.

American Honda Motor Co. Inc., the Japanese firm's U.S. subsidiary, has agreed to provide repairs or reimbursement for a rust condition on the top part of front fenders near the windshield caused by a defect in the fender design, according to the FTC.

The FTC said that Honda "knew or should have known" that the fenders on its Civic and Accord models in those years would prematurely rust. Honda "did not disclose that fact to consumers," the FTC said in a complaint against Honda. The problem was corrected beginning with 1979 models, the agency said.

Honda sold 700,000 Civics and Accords in the four years covered by the agreement. "We would be surprised if the problem exceeds 10 percent of that," said a Honda spokesman, adding that the company expects to spend about $10 million on repairs and reimbursement.

The agreement only covers rust that began on the underside of the fender. It does not cover rust elsewhere on a car, surface rust, or rust due to stone chips or collision damage.

If the replacement isn't made within six months after an owner is contacted by Honda, the owner has the option of receiving $150 for each rusted fender.

Under the agreement, Honda will send notices of the replacement agreement to owners in 24 states and Washington, D.C., where auto rusting has been prevalent because of road salting in wintertime. Virginia and Maryland are two of the states. Owners who do not receive such notices but have cars with rusted fenders should contact Honda to arrange for inspection and qualify for the program, the FTC said. Owners also may contact the FTC at 523-3598.