I need to get rid of the anodizing inside a D cell Mag, but ONLY from the circlip up and nowhere else. I need to do it without grinding/sanding as I need it to stay at the same ID. Is there anything that can be painted on and stay in place and not ruin the ano on other parts?
Easy Off oven cleaner.
Noooo... I've seen what oven cleaner does to aluminum, not gonna do that.
The best luck I've had selectively removing anodize (or poweder-coat) from aluminum is to blast clean the item using crushed walnut shells as the abrasive material.
Everything I know of would be a liquid that would not stay in place. Unless you can mask off the area, I don't know anything non-abrasive.
Google it. There are many that have used over cleaner many times. Just leave it on long enough to fade the finish, not eat the aluminum. Tape off the area to be stripped, spray, remove, neutralize with water, 0000 steel wool, polish.
5-10 MINS IN caustic soda solution,bought mine from boyes store ,crystals only a couple of quid .afew teaspoons in cold water NOT HOT.Then rinse well and polish.
I can't dip it. I need the threads still anodized, and inside the tube below the circlip still anodized. Only the area inside the tube above the circlip needs stripping as that's where the big chunk of aluminum will be pressed in.
You could try masking with thick grease, use a small brush where you want to take off the ano.Mix up solution and see if it reacts with grease if not you have plenty left for next time.
You could try masking it with gum Arabic. That’s what we used in art college to mask aluminum litho plates before applying sulphuric acid. Then it just rinses off with water.
I'm going to try the 'greased lightning' dabbed onto a dime-size area at a time, then rinsing & doing another spot.
I have used caustic soda many times and it works within minutes dipped usually.
What you could try is wet a rag in a strong caustic soda and water solution and turn the mag upside down and wet just the inside of the mag while holding it upside down. Then once wet set it down on it its top and let the caustic soda work. Come back in a few minutes and dry the caustic soda off with a clean dry rag while holding upside down. It may take two or three applications but I’am pretty sure it will remove the anodizing with out having to submerge it. You may want to cover the what ever your going to set it down on with something that it want soak through or use a glass plate.
This can be found at my local Ace Hardware and Lowes. Mix a couple of table spoons in a cup of water for a strong solution. It helps to dissolve the crystals in warm water, stir until mixed well and use some gloves for your protection.
100% (sodium hydroxide) caustic soda.
Could you mix some gelatin into caustic soda solution to help make it stick?
I heard a story of a guy who placed his motorcycle Aluminium brake callipers in caustic soda to clean them.
When he came back 20 mins later, they were gone!
!!! 
Try painting the protected areas with melted wax. It should last long enough to do the job. Don’t leave the deanodizer on until all the color is gone, just until it fades, then rinse and dremel wire brush the rest. I just used tape for this. 
See small solitaire on right.
Wait, what exactly is the purpose of this may I ask?
He must have used a lot of it if this story is true.
I remember a video of someone showing how they removed anodizing from a mag 2D. Can’t seem to find it. Seems like he suggested 30 minutes. That would greatly depend on how strong the solution is.
I have removed the anodizing from a 2D host and it takes around 20 or 30 minutes in a 3 or 4 table spoon to a 1/2 gallon of water solution. I usually just check the piece ever so often to see how well its being removed. If its acting kind of slow I add a little more caustic soda (impatient).
A caustic soda solution dip is the way you normal prepare the aluminum to be anodized. It removes any oxidation, revealing clean fresh aluminum to be anodized. Aluminum oxidizes in open air in about a day or so after formed or machined. It needs to be cleaned first before anodizing or anodized right after machining.
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I have a big honkin' slug of aluminum that's only a press fit if the tube ID stays the same. Being a press fit solves two things at once: no retainer needed and heat transfer.
From what I remember the anodizing has a very negligible effect on heat transfer.
PilotPTK might chime in here, he knows all about this kind of thing.