Forty-third Street, between Seventh and Eighth avenues, long has been a street you ignore. It's just there between the porno dives of once-gala 42nd Street and today's hit houses on 44th. The major traffic is of trucks backing, as they also do on the 44th Street side of the building, into The New York Times.

Up, on a scaffold on the south side of 43rd, two men are sandblasting. Black grime is giving way to clean, warm, rosy bricks above wide exit doors.

This is the rear of 42nd Street's Apollo Theater, opened in 1910 as the Bryant and now a porno palace.

Nearer the east end of 43rd the sandblasters soon will be working on a far more imposing wall, white granite archways over which is lettered "The Lyric Theater". This, too, faces on 42nd Street, where it was opened in 1903 as home of the American School of Opera, a hopeful organization headed by composer Reginald DeKoven. Like the Apollo, the Lyric's lately specialized in porn.

What's happening is that these two landmarks are being turned back to live theater. They will have their entrances, billboards and lights on 43rd Street. First to open will be the Apollo.

The first attraction will be coming from the Kennedy Center, Ernest Thompson's "On Golden Pond" which opens a four-week run in the Eisenhower Jan. 23. It stars Frances Sternhage and Tom Aldredge. Its September production off-Broadway at the Hudson Guild Theater pleased critics from Variety to The New Yorker.

Because almost all New York's theaters are booked, several plays are angling for the Lyric, which will open later.

Cross the blight of 42nd Street and look in on 41st. You'll see what is now labeled the Billy Rose Theater, which has just been taken over by a London group and New York theater owner James Nederlander. Opened as the National in 1921, it's been home to such smashes as "The Little Foxes," "Inherit the Wind" and "Who's Afraid of Virginia Woolf?"

But for some years the 1,200-seat theater has been unused, largely because of its seamy surroundings. Now it will have a new name, the Trafalgar, and will reopen in March with "Whose Life Is It Anyway?" the British drama so admired this fall at the Folger.

Times Square's east side has been in eclipse but here, too, real estate is looking up. Forty-third Street's Town Hall, which has slipped in concert bookings, has been designated a New York landmark. The aim here is to preserve the exterior but revise the inside to accommodate two 500-seat theaters in a plan developed by the Hudson Guild Theater's lively Craig Anderson. Across 43rd, on the south side, is the lovely Henry Miller Theater, now a movie house, but redeemed from the bondage of porn. With only 900 seats, it could be elegant again.

Up on 45th Street, east of the square, New York has just declared the untenanted Lyceum Theater a historic landmark. Opened by Daniel Frohman in 1903, this has a strikingly handsome facade which suggests Grand Central Station before it got its PanAm Building backdrop.

And several blocks of the 40s west of Eighth Avenue are dotted with small theaters, once flicks or even churches.

These theaters will make up for two imminent losses. The Portman firm, which did its best to demolish Washington's National, soon will be tearing down the Morosco and the Helen Hayes, back-to-back on 45th and 46th streets, respectively. The Portman firm has been planning a hotel on these two theater sites since 1968 and now both houses won't last much longer.

These relatively modest redecoration investments reflect some confidence in the longtime theater center. The entire Time Square area still looks like hell itself but those redeemed theaters are hopes for the future.

The reasoning lies in the theater patronage. The '77-'78 season was financially the biggest in American theater history, some $104 million into the Broadway box offices and some $105 million from touring productions. Regional theaters, operating on deficit budgets, account for millions more.

There are no artistic statistics to match the financial ones and so far this season has been even dimmer than last. It's all too simple to see that grosses are higher than ever because ticket prices are too.

A second factor for the confidence is the prevalence of holdovers. A single performance can be more than enough for a flop, but once something has been decreed a success, its lifetime seems limitless. Three, four and five-year runs are becoming ordinary. Their long runs keep the theaters filled but they also keep new productions from opening. Hence, those sandblasters on 43rd Street. New York doesn't want to lose of its major tourist attractions. CAPTION: Picture, In their heyday: the Lyceum, left, and Billy Rose Theaters.