How do I clean old dirty cookie sheets

oven cleaner...spray on. Put cookie sheets in a black plastic bag that’s in a warm are...in summer I set it out side. Wait 24 h. Take pans out of bag. Rinse. They should look a bit better

Once their dirty they are pretty well impossible to clean, buy a new one and use parchment paper on the bottom to keep it clean each time.

Y. Try dawn and vinegar with hot water and allow them to soak before replacement. Also are they non stick ones the dark ones or are they silver shiny ones. If they are non stick you can't scrub with a mettle pad but if silver Shineyones then scrub them with a brill pad. Also try brill the powder for cleaning stainless steel sinks a faucets and see if that doesn't remove the gunk. And if non stick and you either get it cleaned or replaced never use spray oil like Pam on it because forget it that will be almost if not impossible to clean. I would suggest that you use the non stick foil wrap a little cheaper than parchment paper. Which you can use both over gunky pan now. No need to replace pans now before holidays although this is a great time for sales. But I love that foil it's heavier than regular foil but lighter then the heavy duty foil and everything just slides right off and you can if you wish still better it if the recipe calls for it. We use it all the time some of my pans are as clean as they will ever be and I don't feel like replacing them when I know that everyone else in this family just doesn't listen so I personally have chosen to keep them and just cover them with that non stick foil. I have only tried the one major brand so I cannot speak about any other brand. I have used parchment but you have to change it each time BTW batches while you can just wipe this one clean if need be I just reuse it and when I am done for the day I just lift it up and throw away. You can wash it while still on sheet and then remove so that it can be use to clean your grilling grates in the summer.

Try barkeepers friend

Spray with oven cleaner, put in a black garbage bag and set in the sun for a few hours. Remove the pan, toss the garbage bag, rinse well and then wash as usual. It will really help. You can do the same with your oven racks.

oven cleaner or toilet cleaner with bleach. both work well

Stainless steel pan - put in garbage bag with ammonia and set in sun - next day check you will be amazed. I did this with a large roaster pan I bought at garage sale which was so bad and after this - it was like I just bought a new Pan!!!

How about recycling them?

https://www.google.com/search?q=recycled+cookie+sheet+ideas&rlz=1C1BLWB_enUS700US700&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiBlsuu9tnXAhUW-2MKHU-fAzoQ_AUICygC&biw=1422&bih=684&dpr=1.13