the expanding foam is moisture resistant, thats the whole reason its used to seal leaks. i did this foam fill a few wks ago on my mutrol more in a water proofing attempt than dust as there are drain holes in those rear guards, there are pics in my mutrol build thread. i painted the inside cavity first to prevent rusting but moisture can not get trapped between panel as its virtually airtight now. makes for good sound deadening too as it stops the flimsy panels along those guards from vibrating to much..
dust smell could well be coming from your air con / blower "bozza72" as the vent air intake is the grill across the top of your dash/bonnet. if your doing a lot of following others or driving in dusty winds then theres a good change your sucking it in that way, just set your a/c to recirc and it will prevent that.
avoid thinks like silicone based products as they promote rust on bare metals, despite the fact people commonly use it to insulate wiring etc... if you paint and seal the cavity first it shouldnt rust any different as it would without the foam.
You do not have the required permissions to view the files attached to this post.