This stuff seem's pretty good,
http://www.eastwoodco.com/jump.jsp?itemID=1124&itemType=PRODUCT
It all depends on how bad the scale is ,you may want to blast the area, then use the oxisolve ,the good thing about this product is it leaves the surface in a converted "zinc" coating ,if you wan't to get into the acid based product's such as PPG's DX579 ,you need to cut it in with water, it's best to do small areas so the liquid does'nt get the chance to dry on the metal, if you have deep pitting you will have to use a scotchbrite pad and work the area till you get a orange paste ,then wipe the paste off ,re-apply then rinse the area completely ,if the acid get's into an area on the car you can't get to it could be disasterous down the road ,after the complete area you want to treat is finished, you have to repeat the process with DX520 ,another acid that prep's the area ,then coat with epoxy primer ,it seem's like alot of work but after all that it is pretty much bullet proof ,I purchased a gallon of the oxisolve ,I am almost ready for the epoxy on my Mach ,I may be ready this weekend ,I decided to try the oxisolve under the hood ,last weekend I sand blasted that area, it had some scale ,if the oxisolve work's well I'll post the result's ,it sure would be better not to have to deal with the acid based materials.Hope this help's