Most windshield scratches or nicks defy do-it-yourself fixing. And, in fact, fixing at all. Pro-detailers and windshield glass specialists have a "rule of thumbnail": if, rubbing a thumbnail across the scratch, you can feel the scratch, it's probably too deep to fix.
A tiny surface scratch can sometimes be rubbed or buffed out with very fine powdered pumice or with jeweler's rouge (both are available from glass shops). Make a heavy paste using water and pumice (or jeweler's rouge). Spread the paste on and around the scratch. If machine buffing, use a non-abrasive foam buffing pad on a low-speed orbital buffer. Use very gentle pressure while buffing. You may have to re-apply the paste and rebuff several times to buff out the scratch.
Rubbing out the scratch manually involves the same water and pumice/rouge technique, only you use a very soft, non-abrasive cotton cloth as your rubbing tool.
While deeper scratches can sometimes be removed by machine buffing, the result is seldom satisfactory. Although you may rid the windshield of the scratch, deep buffing causes a concave place where the scratch was. Result: Vision through the former scratch area is distorted. Far better to keep the scratch than to cause windshield-and vision-distortion.
On the market are a number of "fixit" kits for reducing the visibility of windshield nicks, rock pocks, and scratches. Generally, the results are not very satisfactory. The patch places are often as obvious as the windshield damage they "correct. In most cases, it's not detailing that an injured windshield needs, but replacement.
Aside from detailing the windshield for visibility and cleanliness, a number of products are available to (I) help keep it clean and clear; (2) reduce fogging or steaming; and (3) disperse rain, snow, ice and sleet. Rain-X, originally named Repcon and developed for the U.S. Air Force to keep jet fighter windshields rain-free, is a wipe-on liquid that quick-dries to coat the windshield with an optically clear, transparent polymer (plastic) coating, which disperses rain, snow, ice and sleet. Used on windshields, it largely eliminates the "vision tunnels" produced by wipers. In fact, it is often not necessary to operate wipers on a windshield treated with Rain-X. The film causes an aerodynamic runoff of rainwater and snow, clearing the windshield without any, or only infrequent, wiper assist-thus its claim to being "the invisible windshield wiper."
Applied to rear and side windows, and on rear-view mirrors, Rain-X provides greater visibility in rainy or snowy weather. It is also effective on many convertible and off-road vehicle plastic windows. The product is not, however, a defogger. Its useful life varies, depending in part on a vehicle's speed and use. If you commute several hours daily at superhighway speeds, Rain-X may have to be reapplied every few weeks.
Snow skiers have long used anti-fog, chemically impregnated cloths to keep their goggles clean. Larger versions of the anti-fog cloths work well on windshields. You should be able to find them at local ski shops. There are also interior window defogging formulas available.
While there are many useful glass cleaners available, from household glass cleaners to auto-specialized pre-moistened towelettes for quick-cleaning windshields, what cleans them about as well as most commercial products is ammonia and water. The mix: 1 part ammonia to 4 parts water.
To rid windshields of stubborn grime, stains, and bugs, many pro-detailers use super-fine 0000 (be sure it's 4-0) steel wool. The same super-fine, non-scratch grade of steel wool is also frequently used by pro-detailers to polish windshields, especially those streaked by hard-water residues.
The nemesis of interior glass and, in fact, all interior surfaces is vinyl vapor residue: the oily vapor given off by vinyl (upholstery, vinyl dashes, other interior vinyls), especially as vinyl grows older, when exposed to sunlight. The hotter the weather-and the hotter a vehicle's interior-the greater the vaporization of the vinyl. Not all vinyls vaporize as readily as others; nor do all give off equal amounts of vapor.
Most people seeing a vinyl-vapor-smudged windshield or windows assume the driver is a smoker. Maybe, but vinyl vapor leaves a far more bothersome residue on glass-and on the vehicle's entire interior- than any dozen packs of cigarettes. In sunbelt states, especially in summer with the windows shut, vinyl residue can come close to coating interior glass in a single day. Not only is the residue vehicle-disfiguring, it is dangerous, limiting visibility.
To remove vinyl residue, use any good household or automotive window cleaner, an ammonia-water solution, or all-purpose cleaner.