If they are that bad, there is probably no hope for them at this point. Instead, get some high quality new ones & line them with foil or parchment paper prior to each use. You could also invest in some Silpats, which are the reuseable version of foil & parchment.
If your cookie sheets are heavily coated with burnt on food, they will not bake evenly anymore & you risk burning more foods, not to mention things will stick to them more. If your cookie sheets are too lightweight, foods are more likely to burn on them b/c they won't heat evenly, creating hot spots & causing your foods to stick or burn to the pan. Even with the really good pans, you should still line them with parchment, foil or a Silpat prior to each use to protect them & help them last longer.
I have four Wiltion professional weight cookie sheets & four Silpats to go with them. I've been using the same cookie sheets & Silpat liners for 15 years.
at 10:34 AM on Oct. 19, 2009