OH. NO!" breathes the anxious Washington hostess as a careless guest rests a damp highball glass on an heirloom table . . . or, perhaps, balances a lighted cigarette on another priceless antique.

But what to do now, after the damage is done?

Authorities here and in New York have various answers.

Clement E. Conger, White House curator and the man at the State Department who has assembled the $19-million-plus Americana collection of anitques has several suggestions.

Of course, most of the tables and desks at the State Department's hospitality suite are topped by glass to protect from harm, but sometimes accidents do happen.

Conger sees to it that all the fine furniture at the John Quincy Adams and the Thomas Jefferson Reception rooms on the eighth floor of the State Department's hospitality suite are protected by periodic doses of "Stawax," which he orders from Stair & Co. in New York. It is a kind of bee's wax that is applied every six months or so.

Harold Sack of Israel Sack Inc. in New York is a great booster of paste wax, too, and recommends vigorous rubbing two or three times a year. He says Johnson paste wax is as useful as those imported from England.

He also says humidity control is vital for antiques, especially for those imported from overseas. "Great damage is done to antiques by a change of climate," he says. "To protect the patina of old pieces, there must be climate control in the home, in winter as well as in the summer. Dryness any time of year is the great enemy of good furniture."

In Washington Raymond Launay of Launay and Co. - who does a great deal of work for collector/writer Joseph Alsop and with Clement Conger's own antiques and those in the State Department collection of Americana - imports Goddard paste wax from England and advises the clients to use it on their antiques every six months. This gives the furniture a base of protection against that wet cocktail glass.

As for marble-topped tables, Launay suggests pumice stone to remove stains from the marble. But he says using pumice is a tricky process, and usually requires professional handling.

He has the same advice for the hostess who finds cigarette burns on her antiques. "That sort of thing needs professional care," he says. "It is almost impossible for the amateur to handle a cigarette burn on priceless wood."

Not all smoking is harmful for priceless wood, although the experts do not always agree. But a tip from the butler at the home of the late Secretary of State and Mrs. John Foster Dulles seems to work.

The Dulles' butler hoarded all ash from cigars smoked at the Dulles home here. He made a paste with it from a small amount of water and rubbed the paste into water stains on furniture made by careless guests.

We asked the experts in New York and here about this tip, but none of them and ever heard of it.

But experience shows it realty works. The catch is: How do you get the ashes?