:welcome:
This is a perpetual problem with just about everything that comes out of the AYP (Husqvarna ) factory when used in a dusty enviroment and made 500% worse if mulching.:thumbdown:
Glad to see you
1) worked it out from first principles , looking

2) fixed the problem :drink:
Now to prevent a reoccurance get yourself a very long dust gun 3' to 4' works very well.
At least once a season, blow those areas where the build up was with air. Three or four times would be better.
Using the long gun you can do it without getting a face full of crud.
Never wash the mower with water except to make the exterior body work look pretty.