Giving films the old soft soap

If you want to make a movie about the Russians, it's the little things that are sometimes hardest to find. A box of Soviet laundry detergent, for instance.

Finnish filmmaker Lauri Torhonen, working in Helsinki as assistant director on the movie ''Gorky Park,'' was asked by the props department for a box of detergent.

The props and set crew had been able to supply Soviet dark bread for shop windows (the Finns bake it themselves), and communist propaganda posters, Moscow subway signs, and even snow (which had to be brought in by truck from 100 miles north for a scene near the harbor because of an unusually mild winter here).

Coming up with Soviet Volga and Zhiguli cars for street scenes was no problem , either. And they had fashioned giant red stars to hoist onto towers of a Helsinki building to simulate the Kremlin.

Even the Ferris wheel in the real Gorky Park had been imitated by another installed in Helsinki.

But laundry detergent?

''That was a tough one,'' Mr. Torhonen said. ''We managed it, though. I have a friend who lives near the Soviet border. He had the idea of going on board a Soviet ship and asking if he could buy some of its detergent boxes.

''The props people were really surprised that I was able to present it to them the day after they'd asked. . . .''

Which goes to show that Finland is a convenient place to use as a set for Moscow or Leningrad.

Finland has other advantages:

* Remote steam-train tracks that can stand in for Siberian scenes.

* Old airport barracks buildings built north of Helsinki during the German occupation in World War II, suitable for use as prison camp buildings in the notorious Gulag archipelago.

* A castle in Turku, which doubled as a gangster's lair in the movie ''The Billion Dollar Brain.''

* Plenty of willing extras.

In ''Reds,'' says production manager Kaj Holmberg, ''We had to find a way of keeping 4,000 people in Helsinki Cathedral Square from 8 a.m. to 8 p.m.''

''We had TV personalities telling jokes on the cathedral steps to keep people amused,'' he recalls. ''We had raffles every half-hour for pocket cameras. We had 250 return tickets to Stockholm on a ferry. And the grand prize was two weeks' holiday in the United States. . . .''