Just when you thought that national promotions built around “free” were beginning to look like invitations to disaster, someone’s pulled one off—and even managed to control the blowback when the supplies of free product ran out.

The campaign is Glidden Paint’s offer of a free quart of paint from their newly re-launched product line, introduced in May. Starting June 25, customers were told they could score a quart of free paint in their choice of color by going to a page on the Glidden Web site, clicking on a color wheel to select your hue, providing your contact information and then waiting for your paint to arrive in the mail.

The giveaway was supported by some serious prime-time media buys for a TV spot that showed everyone from Eskimos and nuns to supermodels and some kind of acid-yellow street performer lugging cans of paint through city streets, through amber waves of grain and across purple mountains’ majesty.

Now, there are many ways this could have gone wrong, judging by recent experiences with “free”. First, the Web site could have gone down from traffic overload (hello, Dr Pepper); it didn’t. Second, the form could have proven buggy and hard to open; it wasn’t. (What’s up, KFC?)

Third, Glidden could have somehow roped balky franchisees into a promotion that actually cost them money–and ultimately cost the company more in customer goodwill when they failed to abide by its rules. (Howzit, Quizno?)

And fourth, Glidden could have run out of the product on offer, a quart-sized container of paint in the customer’s choice. (Ahoy, Popeye’s.)

Oh, wait. They did. But in large part, it didn’t seem to matter.

The company is handling the demand overflow by offering customers who come looking for the free paint a $5 coupon off a gallon of the stuff, redeemable after purchase from any of an apparent multiplicity of Home Depot outlets (if not all.) Users simply print out a rebate form from the Glidden promotion Web page, attach a copy of the Home Depot sales slip and the UPC code of their paint purchase, and mail that off by Aug. 5 to get a $5 price break. There’s even a store locator to help them find the nearest Depot.

I think it’s an effective way to handle excess demand for a giveaway promotion. For one thing, the ads were very clear about saying that the promotion would run out when supplies were gone or on July 2, whichever came first. Well, Glidden apparently made it throughfour days of a potential eight-day promotion. That’s not great demand forecasting, but not terrible either; while I’m sure they wish they’d had a bit more product in place, some shortfall was almost inevitable.

The point is that when that supply dried up they had a solid contingency plan in place. In fact, although I’m not much of a home-repair maven—I’ve been living for 20 years in apartments that are decorated in that color universally known as Landlord White—I’d have to guess that for those motivated to paint, getting a rebate on a full gallon is better than getting a free quart. After all, that’s enough to paint, what? A footstool? A pillar or kitchen island counter?

And not having to wait three weeks for delivery of the free sample is also a plus, to my mind. So again, people left out of the original promotion have a reason to feel satisfied.

And finally, Glidden seems to be doing something especially right. As happens with all free promotions these days, the news has been spread by bloggers and via Twitter. That’s to be expected.

But citizen journalism doesn’t always move at a single steady pace. So at least a day after the Glidden Web site was explaining that the “free quart’ promotion had run its course, Twitterers were still posting tweets touting the giveaway.

Knowing that customers drawn via these posts could wind up frustrated complainers, Glidden seems to have made sure that someone within the company is watching Twitter and posting periodic notices that supplies are gone and the promo’s over, but wait, don’t despair, here’s another offer.

That kind of exit-strategy vigilance is becoming almost as important to a campaign as the initial big idea. Glidden seems to have had the good sense to build in some end-game measures that let the promotion taper off without tears—and maybe even draw in a few customers who find the rebate offer more useful.

Of course there may be another lesson to be drawn here: Come between me and my decor, and I’ll put up with some disappointments. But promise me free food, and you’d better come across as promised–and with a smile.