EVERETT — Paine Field’s main runway reopened Monday night after a $7.6 million project to repair a slippery stretch of pavement.

The 9,010 foot-long runway is the longest of two — and the only one that can accommodate jetliners, including the airplanes Boeing builds nearby. Runway 16R-34L was closed for a week.

The closure was necessary after maintenance work in July. The runway’s surface was mimicking “winter skid conditions,” airport engineer Ken Nichols said. An oil-based sealant to protect the asphalt, applied by an outside contractor, didn’t cure properly.

The main runway also was shedding. There was more loose material than usual, Nichols said. Even small pebbles can cause thousands of dollars of damage to an airplane’s exterior or jet engines.

Although the problem was intermittent — planes were still able to take off and land — it raised a big question, Nichols said: What happens in winter, when the runway is already slick?

Federal Aviation Administration experts and airport officials agreed: Most of the runway would need to be repaved. The airport is owned and operated by Snohomish County.

There was no impact on Boeing’s factory operations, company spokesman Todd Kelley said. “We were able to plan ahead,” he said Monday. Within an hour of the runway’s reopening Monday night, a flurry of Boeing-related flights began to arrive.

Airport and county authorities continue to explore why the summer maintenance didn’t produce the expected results, airport spokesman Scott North said.

About $780,000 of the work is covered by state and federal grants. The remainder is likely to come from previously budgeted airport money that was set aside for work scheduled but never undertaken, North said.

Scraping off the poorly cured sealant wasn’t an option due to the runway’s grooved surface, said Bill Penor, airport maintenance manager.

The grooves, which are about an inch apart and run horizontally the length of the runway, are there to increase braking efficiency and decrease hydroplaning.

The problem is, “you can’t get the sealant out of the grooves,” Penor explained.

The repaving project got under way Oct. 1.

Paine Field’s main runway was last repaved in 2009. It must withstand the weight of the world’s largest aircraft, such as the one-million-pound Boeing 747-8. That requires a higher-grade asphalt than is used for most roads, Penor said.

An asphalt runway usually lasts about 20 years. Paine Field’s two runways accommodate more than 300 flights each day.

More than 100 workers spent the week toiling around the clock, milling and repaving a swath of the nearly two-mile runway that was 65 feet wide and 8,000 feet long — a new surface down the center of 150-foot-wide pavement.

More than 40 dump trucks removed nearly 14,000 tons of asphalt, a two-day endeavor.

Once the old material was hauled away, the paving machines rolled in, followed by ground-shaking barrel rollers to tamp down the hot asphalt.

Except for periods of heavy rain, crews laid down about 400 tons of asphalt an hour. In all, it was the equivalent of paving a single-lane road for 18 miles.

On Saturday, crews re-installed the runway lights.

On Monday, the airport’s sweeper trucks blasted away leftover debris.

Penor promised that the re-opened runway would be “clean enough to eat off.”

By mid-afternoon Monday, the maintenance crew had finished painting new lines. At 7 p.m., the main runway reopened.

Paine Field Airport Director Arif Ghouse was proud of the airport engineers, maintenance people and Interwest Construction, the project’s prime contractor.

“They understood what was at stake and rose to the challenge,” Ghouse said.

One more task remains, North said. In a few weeks, the main runway will get its grooves back.

“They’ll be grinding them into the asphalt,” he said.

Janice Podsada; jpodsada@heraldnet.com; 425-339-3097; Twitter: JanicePods.