IT was James Watt's foot that made the Giannettis famous.

Here they'd been architectural sculptors in Washington for decades, and their father before them, doing fancy plaster work on the Capitol, the White House and all over. For more than a year they've been renovating the Renwick Gallery, a five-year project that they say will cost $15 million to $20 million.

And then the White House staff orders a plaster foot with a hole in it for a gag on President Reagan. But Reagan turns it over to Watt, and the whole country laughs, and Giannetti's Studios is suddenly on the map.

"It's funny how things work out," says John Giannetti, 42. Last year, when he ran for the Maryland legislature, he thought about getting some incidental publicity on the Renwick job but decided not to. Now, as he finishes the very last plaster mold on the fac,ade, the press shows up. The Renwick's sandstone exterior features are melting away, and Giannetti is making molds of every lintel, corbel, molding, medallion, pilaster, keystone and cornerstone on the entire building: everything but the bricks.

The stonework will all be restored to mint condition and reproduced exactly in precast architectural concrete by a Pennsylvania firm. It may be another three years before all the pieces are replaced.

"Those corbels the little stone ends that stick out under the eaves go all the way through to the inside wall," Giannetti said. "It's an enormous job. But the pollution was really destroying this sandstone. You can crumble it with your finger." He gazed up at the venerable landmark. "You know, sometimes, working up there, we'd feel a presence. Someone watching us. Those early stonecutters."

The studio in Brentwood, Md., just over the District line, is cluttered with plaster models of all these pieces, including two 18-foot free-standing fluted columns in sections. The last bit, the large medallion keystone over the Pennsylvania Avenue entrance, was cast in rubber and plaster by Michael Pathlak, a Russian-Jewish emigre' who learned his trade in Kiev.

"We've seen a lot of action on this corner," Giannetti says. "ERA demonstrations, ambassadors at Blair House next door, the president going by. Sometimes we'd have to get down from the scaffolding. The Secret Service would come by and call us: They already knew all our names."

The sculptor and his brother Robert grew up in the plaster business here with their father, the late George Giannetti, who came over from Italy in 1910 at the age of 14. They worked on the Capitol's east fac,ade and interiors, Mount Vernon, Ford's Theatre, the Kenmore House ceiling ("one of the most beautiful in America") and the press office moldings in the Nixon White House, not to mention fancy ceilings in Potomac, McLean and so on. Their father sculpted many of the executive seals around town, including a 16-footer on the Federal Deposit Insurance Corp. building.

As for the foot: No, it wasn't anybody's foot in particular. "They ordered it six weeks ago--with the hole. We did it in Fiberglas. My brother sculpted it in clay in a couple hours. We couldn't talk about it because it was a big secret at the time."

He smiles with the amused tolerance one would expect of a man who has spent 18 months working across from the White House.