I am thinking of having an old bike powdercoated to spruce it up a bit.
What do I need to do to prepare the frame? Remove all components? What about stickers and decals? Some kind of chemical etching dip?
It is an aluminium MTB frame.
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You'll definitely need to remove all components. For threaded bits you have two choices - leave junk bolts in there that will get powdercoated over and then carefully remove them (leaving a bit of an edge around the bolt hole) or let them get masked but almost certainly somewhat powdercoated into and use a tap and die set to chase them before reassembly. Be prepared to face/ream/chase the headtube and bottom bracket in any case.
Do make sure you've cleaned it as much as possible, removing any residual grease and road dirt. I wouldn't worry about the stickers and decals, the media blasting to prepare the frame should take care of that. With the relatively low cost of media blasting I wouldn't bother trying to strip the paint myself.
Make sure you find a powdercoater that is familiar with bicycle frames; particularly when it comes to media blasting they need to understand how thin the tubing is.
I have taken a bike to a powdercoater before. Many of them actually do motorcycle frames regularly so they're very good with prepping the surfaces and screw holes and dealing with complex shapes.
The hardest thing for me was the fork race-- I had to have it removed by a bikeshop. Some people have success with a DIY approach, but it is safer to do it with the right tools and experience (it is not a routine maintenance activity).
If there is residual grease or frame-saver compound in the tubes, the powder-coater should have solvents to remove this stuff. Just ask them about your concerns.
Everything must be removed from the frame. Powdercoaters will do the sandblast/strip for you generally. Alu must be chemical dipped. Good powdercoaters will put plastic/rubber plugs in the screw holes so that they remain clean. It's always good practice to chase the threads when you get it back from the powdercoater.
You indeed need to remove as many components as you can. I took an old cruiser to a powder coater a few months ago and had a pretty nice result. From my experience they will sandblast the frame before so you do not need to worry about it being dirty or having stickers... it will all be taken care of with the sand.
As mentioned in other posts the main issues are the threads and the bearing races. The worse for both would be if they get sanded. Try to cover them as well as you can with duck tape or equivalent and make sure you clearly explain them not to touch those areas. On my bike there was a bit of paint on the bearing races for both the fork and the bottom bracket (old type of cranks/bb). I used a dremel with a metal spinning brush in order to nicely clean the races and reface them, and it seems to be working pretty well so far (make sure you grease them afterwards).
Good luck with it!
Make sure to clean out all grease and oils in the frame tubing. I just recently had my bike painted and while they hung the frame up to dry, heated grease caused a little defect in the frame on the top of the seat post insert.