Back in the early days, automotive paints were tough and thick and about half as pretty. In the 1950s and 1960s, manufacturers started looking for ways to make paint shinier, smoother, more versatile and cheaper to apply. By the 1980s, acrylic paint had all but taken over the marketplace. It was fine for the typical car body but nowhere near tough enough to withstand truck-bed duty. Enter, first, plastic drop-in bedliners and then today's super-high-tech, spray-in liners. These spray-in liners might be a solution to a problem that never should have existed -- but they're leagues ahead of what any paint could handle.
All spray-on bedliners start as a liquid mess of molecules. After application, those molecules link together into long chains of tough polymers, which lay on top of each other like a pile of wet spaghetti. When they fully cure, they do the same thing wet spaghetti does -- they cross-link and form a tough web of strands. There are two basic types: "oxygen-catalyzed" and "chemically catalyzed." Oxygen-catalyzed -- or "single-part" -- bedliners cross-link in the presence of air, like paint, super glue or caulk. They're descended from very thick acrylic house paints. Two-part, or chemically catalyzed liners, are more related to the polyester resin found in fiberglass or the super-tough, two-part epoxies used to make on-the-spot machinery repairs.
The spray-on or roll-on bedliners you find in major stores are generally single-part affairs. They're designed to be cheap, easy to apply for the average person and good-looking on application. But you get what you pay for. Comparing one of these commercially available liners to a two-part liner is like comparing wood glue to carbon fiber. Single-part, spray- or roll-on liners are just this side of purely cosmetic, proving easy to gouge and peel at the slightest provocation. Sometimes that's all you really need. These types of liners work fine for everything but truck beds, where waterproofing and a certain degree of toughness are more important than the ability to carry a jet ski without damage. People have used them for waterproofing interiors, painting exterior body parts and even sealing outdoor ponds and pools.
Two-part bedliners are the serious lining materials, used by people who expect to use their trucks like trucks. They come in "hot application" and "cold application" varieties. Both hot and cold types mix two components that, when they come into contact, begin a process of very deep and thorough cross-linking. The end product is something like the ABS plastic bumpers on many newer trucks. There's no debate that hot application makes for a better-looking, more durable end product -- if you have the budget and time, there's nothing better out there. But cold-application liners are rapidly closing the gap. Cold application requires only a compressor and a special spray gun that mixes the two-part compound as it comes out. It's as easy to apply as thick paint, and the best can offer about 90 percent of the performance of hot-application liners as of the time of publication.
Spray and brush-on bedliners are thick, and they cover a lot of sins in terms of rust, bad paint, body filler and seam sealer. But don't take that to mean you can just spray the stuff down over a grungy bed and call it a day; even the best will peel and chip later if the surface isn't prepared properly. Ideally, you'd use a wire wheel and grinder to get down to bare metal and then prime it with an etching primer. But, if that isn't a possibility, sanding to remove loose and peeling paint and rust and getting a solid surface underneath is a must. You might even consider using the edge of a grinder wheel to make small, figure-eight scratches all over the bed. Any traction you can give the liner will help. After that, you'll need a coat of adhesion promoter; give it about 15 minutes of tack time before liner application.
Realistically, much of any individual's liner choice will come down to budget. If not for monetary constraints, everyone would probably use a UV-protected, Kevlar-impregnated liner. As of 2014, you should expect to spend about $40 per gallon for any product of reasonable quality, up to as much as $250 per gallon for the highest-end products with UV protectants, Kevlar reinforcement and anti-slip or slip-encouraging additives such as Teflon. A small truck like an S-10 might need a gallon or a gallon and a half; bigger trucks might take up to two or three. You'll spend another $50 or so in prep materials and about $80 for the special mixing gun you'll need to apply a two-part liner. Shops and dealers can charge anywhere from $250 to $600, depending on the location and vehicle. At the very least, shop for a liner with a UV protectant; otherwise, you'll find your liner fading after two or three years of sun exposure.
