Can you kosher a public BBQ Grill for your own use in a park? What if you want to buy a used Grill?

In theory it is possible to use a blowtorch (after cleaning off surface grease) and bring to a high glow, but it is generally impracticle at a public place. A use grill is also not easy to kasher but it can be done. You do need a blowtorch and some knowledge.