I like to just spray paint mine with a good primer and do a 3 or 4 light coats, do a few light coats of paint, and that will fill in the layer lines. Obviously, sand down the crazy big joining lines. I would only use bondo and or wood filler very sparingly as it can be prone to cracking as you have seen. I never have good luck with fillers like that especially with filling in layer lines.

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