With regards the paint/DG issue :
Oil based paints are classified as DG due to their flammability risk(Class 3 Flammable Liquids)
The determination as to whether water based/acrylic paints (or any products for that matter) are restricted as DGs is determined by whether anything in it's chemical makeup presents a significant enough risk. As mentioned in another posting, reference must be made to a Material Safety Data Sheet (MSDS) to determine this.
With regards your commentary about corrosion - this is more likely a problem with the metal container rather than the paint having corrosive qualities. There is generally not enough corrosive chemicals in paint to cause it to be restricted as a DG.
Using another example - a popular brand of soft drink has a corrosive as part of it's formula. In it's pure undiluted form, this corrosive is regulated as a dangerous good. However when mixed to form the soft drink, there is not enough corrosive to have the product restricted as a DG. (It is however corrosive enough to do a good job of cleaning coins!)
Hope this sheds more light on the paint issue.