You need to use a symbol fill style for that. Take a look at CC3/Examples/Cosmographer/Traveller 100t Scout.FCW and use LIST on the deck area with the grid. You'll see that it's two polyongs: the white background, and a second one with the fill style "Square grid w gap, 1.5m". If you open the fill style dialog, you'll find that fill under "Symbol Fills".
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