Tim
If you understand that those paint stick/ crayon marks were applied directly on bare steel B4 any primer, then sprayed over. The image is kind of locked in after stripping. However media blasting is not real forgiving if trying to save these for documentation.
On a car like this Z28, I would have chosen to use some stripper and or lacquer thinner to slowly do the fire wall. This way allot of the primer base is left in tact.
Here is the "N10" notice all the gray primer that bled through the caryon mark.
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And the Big yellow "OK" over top of "Z21"
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Mike