Overall it looks a very impressive model. The camo and seats look excellent.
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but it's hard to make out what's holding the paddle on, as well as the shovel handle.
I think you have painte the molded on clamps the same as the paddle and shovel shafts. Could be touched yo with base colour again.
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I think I would have shown more dried mud on the wheels and under carriage, but that's me.
That could be one solution to make it looked used. Or you could weather the bottom and underneath more ... with dark browns to show that it has been used. Also the engine looks very impressive but the engine compartment is a bit on the clean side. I personally would darken the whole inside and add a few oild stains. A little chipping arounf the bottom rim for the engine cover would be a nice touch also.
Some personal items could be addded to the interior to give it some life. This brings in another dimension to the model. Check out Graywolf´s gallery for his Schwimwagen with added personnal gear, to see the difference it makes.
None of these points take away from your already great work ... just add a bit more realism to it IMO. Cheers for the beers!