The term stainless steel is one of those deceptive little phrases that causes virtually nothing but confusion. What makes stainless steel special isn't the steel, and it certainly isn't stainless. Stainless steel can rust, discolor, and lose its luster and appearance over time. There are dozens of types of stainless steel and each has its own unique qualities. Likewise, there are a ton of stainless steel gas grills on the market, so you should beware of what you are getting by looking at the grade and thickness of the steel, among other factors.
Grades 304 and 430 Stainless Steel
The most common grade of stainless steel (about 50 percent of the world's production) is grade 304. This is used in everything from cars to cookware to buildings. Grade 304 steel is non-magnetic and is prized for superior corrosion-resistance. Some grills and grill parts are made with 304 steel, but many more are made with 430 stainless steel, which is magnetic and is not as corrosion-resistant as 304.
One way to check for steel grade is to put a magnet on it. If the magnet sticks, it's probably made with 430 steel. This isn't necessarily a deal-breaker, and many grills contain some or all 430 steel, but it is one indication of overall quality. Manufacturers often use a mixture of different metals in the same grill so be sure to test all parts of the grill with the magnet if you need to know.
Stainless Steel Grill Thickness
You don't need to take a pair of calipers with you to buy a gas grill, but there are a few ways to get a good idea of the thickness of the metal used. Thicker metal is better and will not discolor as fast as thinner stainless steels. You can tell the thickness of the metal used on a stainless steel gas grill by the overall weight of the grill and how hard the metal feels. Don't be afraid to press the grill in the middle of the hood. If it doesn't budge, the metal is probably pretty thick.
Stainless Steel Finishes
Most rust gets its start in imperfections in the surface of the metal. Tiny holes let moisture and chemicals sit and eat away at metal. Stainless steel, though resistant to corrosion, isn't immune to these problems. When looking at a stainless steel gas grill, feel the surface; it should have a clean finish to it. There are several finishes to stainless steel, but poorer quality stainless should feel uneven. This metal is raw from the factory and not finished. A finished surface may be brushed or smooth, but it should feel even.
Assembly of the Parts
Stainless steel needs to be properly assembled to make the most of the metal's corrosion-resistance. Spot welding—joining metal parts with a series of small welds—leaves the parts vulnerable to corrosion in the joint. A better construction method is continuous welding, and the best stainless steel grills have all continuous welding. This is an expensive process and isn't available on lower-end grills. Grills may also be assembled with bolts or even rivets rather than welds.
What you want to look for is grill parts that come together nicely and are welded or are fastened with stainless steel bolts. It is best if the bolts are 304 (or 316) stainless steel and are not magnetic. Avoid grills assembled with non-stainless steel bolts, which will rust quickly. Generally, pop-rivets are a bad idea, and riveted grills should be avoided.
If you end up with a stainless steel gas grill, you need to take care of it. Keep it clean, keep it covered, and keep chemicals away from the metal. Lawn fertilizers are very corrosive agents and can ruin even the highest quality gas grills.