What model car are We talking about?
IRS or swing axles..??
but cleaning the transaxle ,
If there is a lot of gunk on it, scrap it off with a thin paint
scraper or screwdriver [will take longer]
when You have most of it off..
then spray degreaser all over it and wait a few minutes
then use a cheap paintbrush to disolve any grease left...
the cheap degreaser is OK so long as its the foaming type
get it as clean as possible using the degreaser and spraying with hose with nozzle..
then use a cheap wire brush in a drill...
electric or cordless..
once its clean... another spray with degreaser and water will clean it ready to paint...
You can use a cheap undercoat or etch primer and then paint it with the same brand of paint...
as its not in the sun, it will not fade...
or to paint it properly, use auto spray cans of undercoat or etch primer [and top coat] like Motorpro or Auto one brand..
this paint will last a long time and will look good every time you lay under the car.... lol...
If You want to polish Your transaxle...
You will need some serious cleaning and polishing and many many hours of polishing....
this IS a Big job...
Lee
http://community.webshots.com/user/vw68autobug
here is My yellow transaxle painted about 10 years ago
and My new grey one painted about 6 months ago.. Lee