I am doing a few jobs on my K1100LT and need a bit of advice for the following problem.  The aluminium of the final drive housing and swinging arm have corroded in the past, and a previous owner has painted over the problem.  The paint has now lifted in a few places, and has a very uneven surface almost everywhere; in short it looks a mess.

I have sanded back the paint in some areas and the aluminium underneath is quite chalky and powdery.  I would like to know how best to treat it so that it stabilises and provides a decent surface for paint.  Incidentally, the engine & transmission are painted black.  I do have some brushable etch primer which works well on good surfaces, but is it OK here if I sand off the powdery corrosion?

Having been involved in aircraft structures for more years than I care to remember, I know that there is only one solution. Remove the corrosion by grinding or cutting. A flapper wheel would probably work. Get hold of some chromate treatment and coat for 30 minutes then wash with water. Works for Harriers and Tornados. Apply some 2 pack primer and paint.

Thanks for that, bikerboy.  A quick search turns up something called Alocrom 1200; all I have to do is find a supplier of the 1kg packs.  No mention seems to be made of price; I am guessing that because it is used principally by the aeronautical industry, it will not be cheap.

Not sure about the price these days but alocrom 1200 is what is used on all bare aluminium alloy prior to painting on all military aircraft.
Do you know anyone in the RAF? Marham perhaps?

Remove paint with stripper or soda blasting ect: Then as bikerboy say's, the alloy will have to be chemically etched, these products may be available where you are. Prep alloy with Alumiprep 33 cleaner then use Alodine 1200 to etch . this will leave the alloy a golden colour that can be left as is , or painted over. Just make sure to follow all safety data sheets and instructions, and neutralize products after use as this is acid etching.

The description of Alumiprep 33 indicates: Use to clean, brighten and transform badly corroded aluminum to its natural appearance.

Has anyone used this product for a general cleaning of the non-painted cast metal parts of the motor and drive train?

I haven't found anything that restores the non-painted to like-new condition without wire wheel abrasive methods.

tgolden wrote:The description of Alumiprep 33 indicates: Use to clean, brighten and transform badly corroded aluminum to its natural appearance.

Has anyone used this product for a general cleaning of the non-painted cast metal parts of the motor and drive train?

I haven't found anything that restores the non-painted to like-new condition without wire wheel abrasive methods.

 This is what it's used for a  cleaner ,diluted with 3 parts water, with a scotchbrite pad and rubber gloves.
Just make sure you polish or treat the alloy after to protect it, as this will leave the surface more exposed if untreated afterwards.
Alumiprep 33 is a cleaner for alloy,in  preparation of alodine products for etching or painting, a bit like using deoxidine for bare metal,  prier to etch primer. 
If you paint untreated alloy, that has had corrosion no matter if you use abrasives to remove the corrosion, there will still be traces of corrosion that will return under the painted surface in the future, that's why you should use alodine or related products before painting. Did you ever wonder why the inside of the engine block doesn't suffer corrosion problems? That golden colour on the inside of the engine block is etched alloy, and the upper sump is powder coated over etched alloy.