It's not surface planer marks as you see the actual surface (top left, bottom) are above this wavy surface (you don't run sign foam thru a planer anyway). Depending on how you made your toolpaths (carved letters seperate from pocketing larger space, i'd say your stepover on this toolpath is too large. You would think the letters would show the same pattern but they don't.
Words of Wisdom:
“Words that sink into your ears are whispered…… not yelled”
“The biggest trouble maker you’ll probably ever have to deal with, watches you from the mirror every morn’n”
“The only difference between a rut and a grave is the depth”
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Just remember...when it's time for the hearse to pull up..there's no luggage rack on top!
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The beauty of the Second Amendment is that it will not be needed until they try to take it...Thomas Jefferson