Mike Wiltshire, dragging a bundle of hoses across a sweltering roof, explains some basics: Move smoothly, maintain your equipment; a heavy wind is your worst enemy.
He is enveloped in a paper-thin safety suit, dark glasses, a baseball hat and crusty tennis shoes wrapped in duct tape. The outside temperature is approaching 100 degrees.
He lowers his spray gun into firing position and squeezes the trigger. A chemical batter hisses out, rising like a pancake on a hot skillet when it hits the roof.
It's not complicated, but it can take years to learn how to put down an even coat of foam.
"You have to be careful," warns Wiltshire, 47.
Rocket scientists concur.
Spray-on foam is the main reason that NASA has indefinitely grounded the space shuttle fleet. The insulating material keeps coming off the shuttles' massive external fuel tanks.
A 1.6-pound chunk damaged the shuttle Columbia's wing during liftoff in 2003, leading to the spacecraft's destruction as it reentered Earth's atmosphere 16 days later. Another piece came off during Discovery's launch July 26 but did not strike the spacecraft.
Despite spending $1.4 billion since the Columbia accident to make the shuttle safer, NASA scientists have not resolved the foam problem.
Long before spray-on foam became a Space Age material, it was used on planet Earth to make boats float, keep hot tubs hot and insulate oil tanks, walls and roofs. The simplest version, for do-it-yourself crack filling, comes in a 16-ounce can available at Home Depot for $4.97 plus tax.
Foam on the shuttle tank "isn't fundamentally different from what you spray in the wall of your house," says Sidney Perkowitz, a physics professor at Emory University in Atlanta and author of "Universal Foam: Exploring the Science of Nature's Most Mysterious Substance."
Wiltshire has never worked on the space shuttle, but he has sprayed a few thousand roofs. He's happy to guarantee his work for 10 years. But doing it right, that's the catch.
"Foam is tough stuff," he says, "but it is a little bit finicky."
Atop Goddard Middle School in Glendora, Wiltshire lurches backward, spraying his foam gun left and right over the flat rooftop.
The gun is hooked to two bundled hoses that sit on his shoulder like a boa constrictor. The lines wind behind his back, across the roof and down one story to two chemical tanks in his truck.
Line A carries a chemical compound known as a diisocyanate. Line B contains a chemical mix whose main component is a polyol.
A and B merge at the tip of Wiltshire's gun.
A hot reaction starts immediately.
Wiltshire says a classic foamer's prank is to squirt a dollop of the mixture into somebody's back pocket and watch him drop his pants.
He can spray a strip about 4 feet wide in one pass. A good applicator can lay down a 1-inch coat as smooth as a milkshake.
In less than a minute, the foam is strong enough to walk on. You can carve it with a knife, but it's flexible enough to dent slightly with a hard press of the thumb.
There are plenty of ways to mess up a foam job.
You can't spray in the rain or when the temperature dips below 65 degrees. A drop or two of sweat can cause a blister on the surface, Wiltshire warns.
Applied in tight spaces, expanding foam can break a plaster wall. If the chemicals are contaminated or the pumps malfunction, the foam may never cure and instead remain a gooey blanket.
A smooth, clean job depends on the skill of the sprayer. You can train a decent foamer, but experts say the best ones are born with the talent.
"You move to a certain rhythm," says Mason Knowles, executive director of the Spray Polyurethane Foam Alliance, an industry group in Fairfax, Va. "People who are good dancers are good applicators."
NASA has taken the craft of foaming to new technological heights.
The shuttle tank is a foam job like no other. The insulation prevents ice from building up on the outside of the frigid external tank -- falling ice could strike the shuttle and damage its insulating tiles.
It also shields the fuel from extreme heat as the spacecraft accelerates. Some parts of the tank hit 1,200 degrees.
The 154-foot-long tank is actually three cylinders stacked end-to-end. The rear tank holds liquid hydrogen at 423 degrees below zero. The top tank contains liquid oxygen at minus 297 degrees. Between them is the "intertank."
Each tank is upended and foamed individually. Temperature and humidity are carefully controlled.
A computerized spray gun inches up as the tank is slowly rotated. The foam goes on in overlapping spiral stripes, like a barbershop pole. It is a third of an inch thick, on average.
The robotic sprayer can insulate only the smooth body of the tank. The struts, ducts and ramps that protrude must be foamed the old-fashioned way: by hand, using a polyurethane mix that is virtually identical to that used on roofs.
Foam has primarily come off from those irregular spots, NASA says.
Roofers are not surprised. Wiltshire points out the metal strips that form the edge of the school roof in Glendora.