There are several thicknesses of magnetic material on the market, and just like most things in life, one is better than the others. But that's not all that goes into selecting a proper magnetic sheet for your car.
Magnetic sheet is measured in mils, or thousandths of an inch. Common thicknesses are 15 mil, 30 mil and 60mil.
15 mil magnetic sheet can be found at craft stores, and is great for things like fridge magnets. It's cheap, readily available, and comes in manageable sizes. While these traits are great, there are downfalls when considering such a light sheet for autocross use. First and foremost, the thicker the sheeting, the more holding power it has. Being so light weight, 15 mil sheeting tends to become unstuck with little force applied to it. Additionally, it wrinkles easily, which leads to edges that don't sit flush with the car, which leads to air getting behind the magnet, which leads to missing magnets at the finish line.
60 mil magnetic sheet is pretty much the thickest magnetic sheet available on the market. It's so thick that it can only be ordered in short rolls! While 60 mil sheets have the strongest holding power, they lack in other areas. Being so thick makes them less than ideal for autocross numbers. They aren't the best at conforming to curves common on cars (especially sports cars). 60 mil stock is also difficult to cut into complex shapes, such as numbers, using typical methods. Finally, it's so high-priced, numbers would no longer be economically priced. 60 mil sheeting is best reserved for industrial applications.
You guessed it! 30 mil magnetic sheet is the industry standard for autocross digits. It fits the sweet spot where durability, cost, and holding power meet. Generally speaking, it won't lift at speed, and kept flat when off the car, it won't wrinkle. At Autocross Digits, we use 30 mil magnetic sheeting.
Magnetic film is available in raw magnetic material, as well as with a coating over it. By default, raw magnetic material is weatherproof, but if equipped with a coating on top, outdoor use can become prohibitive. Be aware that many lighter-weight magnetic sheets, as well as some thicker brands are supplied with a paper-based coating. Obviously paper won't stand up to wet conditions. Look for a magnetic sheet with a PVC or vinyl-based coating. Not only will it be weather proof, it will add reinforcement to the sheet, making it harder to tear. At Autocross Digits, we use magnetic sheeting with a satin PVC coating, which is easy to clean, resists wear, and adds reinforcement.
This one is pretty simple. Magnetic sheeting comes in adhesive backed, and non-adhesive backed. Adhesive backed sheeting is good for crafts in hobbyist uses, and for things like attaching to white boards in industrial uses. For autocross numbers, stay away from adhesive backed sheeting.
So there you have it. Now you're an educated buyer when it comes to getting the right magnetic sheeting for your car. Of course, if you're purchasing from Autocross Digits, you don't have to worry about getting low quality or an inferior product.