Try: Soak the duct tape in a few sprays of WD40 and if you can spray anybeneath the tape, that would be best.
Start peeling the tape with a scraper or a putty knife or a blade orby hand.
Spray more WD40 over the peeled off duct tape and let it soak for 5minutes or so and then scrape some more.
Optionally If you have any SOS Pads or steel wool, consider scrubbing the soaked duct residue with it as well.
lifehacks.stackexchange.com/users/16048/SlickNutz
Heat it by leaving it in the sun, pouring hot water, or else heat gun/hair dryer.
This will soften the adhesive and allow you to peel off the tape.
Remove residue with non-polar solvents (anything oily).
lifehacks.stackexchange.com/users/17498/Nissim Nanach
Heat the tape and use a paint scraper or try to peel it off the door, if that does not work, then pour hot water and try then.
Also, if there is some of the glue left on the door, take another strip of tape and repeatedly put it on the area with the residue and then take it off again.
lifehacks.stackexchange.com/users/18127/JSASCS
I have used brake cleaner to very easily remove glue residue.
You can get it at home depot or an autoparts store.
lifehacks.stackexchange.com/users/18292/sherryvette
After removing the layer of the duck tape by heating it with something like a hair drier, use dish washing sponge and warm water and baking powder to remove the adhesive residue.
lifehacks.stackexchange.com/users/10609/Karel Capek
I will suggest you a serious of methods.
All you need is a blade , spray bottle,oil and some kerosene.
By blade, try to peel out edge of tape that is stuck on door.
If you can get a spray bottle , take some oil and water mixture.
When you start peeling off from door, do not try to give it a full shot.
Because, tape may break and you will have to do step 1 again.
Instead apply force randomly.
In between, if you find it tough to pull, soak adhesive part of tape with liquid taken in spray bottle and wait for it to take action.
In the case of oil, I would prefer you to use vegetable oil that you use in your home as a common lubricant.
After taking away entire tapes, you can give door a nice rub with kerosene to remove left back adhesive particles.
lifehacks.stackexchange.com/users/10930/MANEESH MOHAN
If the plan is to clean and paint this old rusty door I would use a hand held propane torch and burn it off.
Then scrape or wire brush it.
lifehacks.stackexchange.com/users/19519/Robert K Arundale
I need to remove large amounts of colored duct tape from an old, rusty door.  What is the cheapest and easiest way to accomplish this?
How can I remove large amounts of duct tape from a door?
lifehacks.stackexchange.com
<cleaning><home><efficiency>
lifehacks.stackexchange.com/questions/13631
lifehacks.stackexchange.com/users/15944/Laura Maxwell
Soaking the duct tape with some WD40 or hot water will help, as will applying heat or leaving it in the sun. Then, you should be able to use a scraper or a putty knife to remove it. Brake cleaner may also be effective. If the plan is to clean and paint the door afterwards, you could burn the duct tape off with a torch and then remove the remaining residue with a wire brush. Soak the duct tape with WD-40 and peel off with a putty knife and use steel wool to scrub off any residues or heat the door to soften the adhesive then peel off the tape (or use a scraper to remove it) and remove residues with non-polar solvents, use a brake cleaner, lift bit by bit whilst spraying with vegetable oil or burn it all off with a propane torch and then scrape with steel wool.
Soak the duct tape with WD-40 and peel off with a putty knife - use steel wool to scrub off any residues. Heat the door in the sun, with hot water or a hair dryer to soften the adhesive and peel off. Use non-polar solvents to remove residue. Heat the tape and remove with a paint scraper or by peeling. Use other strips of tape to remove residue. Use a brake cleaner. Use a blade to lift the end of the tape and keep spraying small amounts of vegetable oil from a spray bottle as it comes off. Once removed polish with kerosene to get rid of any residues. Burn the tape off with a propane torch and then scrape or wire brush it.