I did this because I had bought some painted models from a friend that I wanted to repaint. I was lucky and the models were not primed, as this makes the process more difficult. I also had some Tau models that I had painted about 4 years ago, and wanted to paint them again. I couldn't just paint over the old paint because when I had first painted the models I used way too much paint. The first coat of paint had filled all of the cracks in the model and obscured most of the detail.
Step 1: Materials
Step 2: Soak the Models in Simple Green
I needed about 750ml of cleaner to cover half of my tank, because I could flip it and soak again. I used a ratio of water to cleaner of 2:1. The soak time estimates in this guide are using this mixture. Soak the models in a tub or bowl that you dont care about.
Completely submerge you models in the cleaner for about 24 hours. The longer the models soak the easier the paint comes off. You will be able to scrape the paint off after only 3-4 hours, but it will be much easier after 24 hours. Leaving the models in for too long should not harm them. I left a few in for 2 weeks to make sure, and they are fine. Simple green also weakens dried super glue. Any "green stuff" will easily come off after soaking. The picture below is after soaking before any scrubbing. I wrapped the tub in plastic wrap so the simple green wouldn't smell as much.
Scrub the models right after they come out of the cleaner mixture.
Step 3: Scrub the Model
Don't leave the models out of the cleaner mixture before taking all of the paint off. The paint will dry back onto the model after a few hours.
Step 4: Pick at the Model
Step 5: Your Done!
You can now repaint the models (see below), or sell them for much more than you could a model covered in paint.
If you want to go one step further you can also take apart some models. While simple green helps, I found that putting models in the freezer really helps with taking apart models made of metal. The glue becomes brittle and the model easily comes apart. I also tried this with plastic models, but im not sure if the weakened glue was from the simple green or the freezer.