It depends on what you’re working with. Shoe spray doesn’t inherently damage your shoes and boots, but applying it and waiting for it to dry is a totally different conversation. Whether it’s water repellent spray or you’re applying this for leather shoe care, this is what you need to know.
Canvas - Canvas is durable regardless of temperatures, so using a hairdryer won’t be an issue here.
Textiles (nylon, wool, polyester) - These will be able to handle the heat fairly easily, and you should have no problem using your hairdryer on them. Textiles often come with a bit of stretchiness to them, so unlike thicker materials, the heat will absorb and dissipate fairly quickly.
Rubber - You probably don’t have full shoes made of rubber, but your outsole is bound to be rubber. Concentrated heat from your hairdryer won’t severely damage the rubber, not right away. Over time, rubber breaks down, and while the heat will accelerate deterioration, it won’t warp or damage your sneakers immediately.
Synthetic fiber - You’ll see these in a lot of Under Armour shoes, and this is a judgment call on your part. Because the heat will penetrate the breathable materials, your rubber insole will take some damage. This material is probably the quickest drying (in our experience) when it comes to the dreadful wait after applying shoe protector spray.
Leather (top or full grain) - This is where you hit a dead end. They take far longer to dry than other materials, yet you cannot, under any circumstances, use your hairdryer to speed up the process. Leather has an all-natural set of oils trapped inside, meaning that intense heat will dry them out. This is how you get fading (the unattractive kind), splitting, flaking, all the bad things that leather lovers don’t want to happen.
Suede - Avoid using a hairdryer at all costs. Suede is a type of leather, and while it has less locked-in natural oils than full or top grain leather, you’ll still be destroying them with that the blast of concentrated heat from a hairdryer.