PUBLIC RELATIONS manager Gary Perino calls Grand Union's experimental stores "a theater for food." The language may be overblown, but only the most jaded customer could fail to be cheered by the light wood paneling and extra-wide aisles at Rockville's Grand Union Food Market. Other amenities are the spiffy orange carts, with wheels that operate properly (no need for test driving), and the plastic produce bags, which are sturdy and easy to tear off.

Other signs of Grand Union's determination to change the sullen "take it or leave it" image of the typical supermarket are the staff's boaters and brightly striped aprons, the latter emblazoned with the message, "Ask Me. I'm Here to Help." You won't find telephone numbers listed for other Grand Unions, but in both the Rockville and Belleview stores, you can call not only the manager See GRAND UNION, L2, Col. 2 FOOD MARKETS GRAND UNION, From L1 but each department to ask questions or place an order.

The seafood counter, which store officials say takes three hours to set up each day, is resplendent with fresh tuna, frog legs, monkfish, bluefish and salmon steaks. Nearby are tanks of Maine lobsters and rainbow trout. On request, seafood manager Mike Wood will suggest a couple of sensible ways to cook softshell crabs or shark. Anyone still in doubt could consult one of the recipes distributed, though these are marred by cutesy names (example: "Shrimply Delicious Swordfish Soup") and too many convenience ingredients, such as canned cream of shrimp soup and "packaged seasoning mix."

The bakery counter is also attractive, with an appetizing variety of cookies and cakes, including an endearing carrot cake that one customer described as "a little lopsided, like a homemade one." Unfortunately, some of the "from scratch" baked goods are not up to scratch. The rugelah and apricot-filled cookies recommended to me were lackluster, and the baguettes could use some French lessons.

The day I was there, the cheese department was offering double gloucester at $4.49 a pound, blarney at $3.59 and pre-monde at $3.89. Explaining that pre-monde is a low-salt, low-cholesterol cheese that resembles a mild cheddar, Mary Trevino offered to cut it to order if the prepackaged portions didn't please. She also measures and grinds coffee beans, and alert customers learn to ask for a free sample of the steaming coffee at her counter. Brewed from a different kind of freshly ground bean each day, it beats the coffee offered in a more conspicuous location, which is made with Grand Union's brand.

Except for a few standbys like cole slaw and potato salad, the salads at the deli counter are made in the store. One is a colorful melange of Italian shells, snow peas and red peppers, while another combines broccoli with red onions and tomatoes. The store also roasts its own beef and bakes its own hams.

The meat department offers ground sirloin, fresh turkey breasts and ducks, turkey and duck livers for making pa te', prime cuts of lamb and veal, and game in season. Best of all, it's full service. The butcher will slice calves' liver as thin as you please, or spend five minutes debating the merits of dividing a rabbit into six pieces or eight.

The Belleview Grand Union is newer and even better than the one in Rockville, with a similar decor and the same predilection for cute names. The "Cheese Shop" groups about 175 kinds of cheese by country, including double-cream cheeses such as Saga Blue and St. Andre'. At the "Corner Deli," good smells emanate from the herb-roasted cornish game hens, roast ducks, vegetable lasagna and barbecued short ribs. You can buy Frusen Gladje' ice cream by the cone or pint at the "Nice Cream" counter. And the "Cook's Harvest" counter sells grains, legumes, nuts, tea leaves, whole-wheat fig newtons and other goodies. Customers unsure of what to do with triticale flour or buckwheat can consult a reference catalogue kept at the counter or pick up free brochures.

The produce section is about 20 percent larger than a normal Grand Union's. Here you can find swiss chard, fresh raspberries and blackberries in season, carrots with tops and several sizes of grapefruits. Freshly squeezed orange and grapefruit juices are available, too. Still, the experience does not overwhelm.

The seafood, meat and bakery counters are equivalent to those in the Rockville store.