I used some liquid stuff from drycleaning you mix with hot water, it's removing oil product for work clothings, I took my pipings out and put them over night.. next day all the oil and dirty stuff came off nice..

 

Plain old gasoline works great at cleaning out the oil inside the intercooler.

 

after 2 hrs of sanding
i used 430 (dry) to get rid of all the dirt, grease ...
than to finish it up .. i used 600 (wet) with a lil bit water... and to remove the fine scratches i used 900 (wet) with water as well