ERTL Flatcar
Before and after adding the Modeler's Choice laser cut deck

BEFORE. The deck that comes with the car looks like plastic
and requires a lot of effort to represent wood.

AFTER. Adding the Modeler's Choice wood laser cut deck changes the appearance
of the car instantly. You can easily stain the deck a darker color to represent
an older weathered deck.

Remove the existing deck by unscrewing the 2 screws on the underside. Not the truck screws but the other two.
Glue the underframe to the underbody and glue our deck to the top of the underbody.

Photo shows various shades of weathering for a prototypically correct varied look.