So I had some one ask me how is it when working with foam in building a plug for fiberglass you keep from burning through the Bondo and hitting the foam?
The answer is pretty simple. Use a "China Marker". AKA grease pencil.
What I do is put on the first coat of Bondo. Get it roughed in and mark it with the china marker. As you sand it will show through the Bondo when you hit 0.003 inch. That tells me when to stop sanding and add more Bondo where it needs it.

Here is what it looks like after I add Bondo.


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