Works way better than Windex!
Four plastic models, primed black, basecoated Adeptus Battlegrey, and then painted with Fenris Grey, were placed into a small container filled with Simple Green. The models were fully submerged.
After 4 hours, I checked on the models to see how they were doing. I could see that the Fenris Grey was easily coming off, so I pulled one out and gave it a scratch. All the paint except the primer easily came off. I then pulled out each one and used a toothbrush to completely expose the primer, hoping this would help take the primer off. The models are now back in the Simple Green where they will stay for another 20 hours, unless the primer looks ready to come off sooner.
Like the other models, these ones had painted sand glued to the base with PVC glue. This came off with little trouble after the 4 hours.
I found the Simple Green at Canadian Tire, and payed $5.24 for a 650ml bottle. There was a larger bottle, which lowers the amount of money you pay per ml, but I didn't want to spend more on a bigger bottle until I knew it would work. I'll definitely be picking it up tomorrow, since the 650 ml size doesn't provide a whole lot.
From what I've heard, the solution can be used 2 to 3 times before it loses its effectiveness.