Paint thinner doesn't work in this case. It will ruin the heatsheld.
My way was.......
The fastest and easiest way to remove the paint from the heatsheld is, dip it in Brake Fluid (yes, car brake fluid). But DO NOT DIP IT TOO LONG, should be no more then 15 minutes. Then use a paint brush and lightly brush off the paints while the heatsheld's in the break fluid container. After 10 minutes, start brushing lightly.
Steps to follow:
1) Tape the heatsheld on the side of the white skull entirely. Try to find the kind of tape that is use for water tight or oil resistant. I've used 3m tape, but I put lots of layer. I'd put it on almost 2mm thick, make sure to rub it on good so no bubble appear. The thicker the better, so the oil won't get to the skull side.
2) Get yourself a small bottle of brake fluid and dump it into a little container (deep enough to cover the entire heatsheld).
3) Place the taped heatsheld into the container. Set your stopwatch, make sure you keep a close watch. It's best to do it outdoor.
4)After 5-10 minutes, start brushing lightly.
5) After 15 minutes or after the paint is off (which ever is first) clean the heatsheld with water and soap. If you keep it any longer, the heatsheld start to cracks and become soft. I've found out the hard way.
That's what I've done with my custom 1/48 Kakizaki.