I had a discussion in another thread where I claimed that Canon's non-white weather sealed L series lenses needed a front filter to be rain and dust sealed. These lenses are:
EF 14mm f/2.8L II USM
EF 16-35mm f/2.8L USM +needs front filter
EF 16-35mm f/2.8L II USM +needs front filter
EF 17-40mm f/4L USM +needs front filter
EF 24-70mm f/2.8L USM
EF 24-105mm f/4L IS USM
EF 100mm f/2.8L IS USM Macro
EF 50mm 1.2L USM +needs front filter
EF 85mm 1.2L II USM
EF 24mm 1.4L II USM
Someone disagreed (someone always does)...
(I also noted this on the list above for a quick full reference)
Your parenthetical statement suggests that in your discussion on that other thread you were suggesting that the listed lenses need a front filter to complete the sealing. If so, I'd certainly disagree with your list, if for no other reason than you were suggesting that the EF 14mm f/2.8L II needs a front filter...that lens cannot even take a front filter because of the bulbous front element.
But it's pretty clear which lenses require a front filter to complete the sealing, even if some Canon reps aren't knowledgable about the issue. Just check the instruction manual, if a filter is required the first page states that explicitly: "Since the front element of this lens moves when zooming <or focusing, depending on lens>, you need to attach a Canon PROTECT filter sold separately for adequate dust- and water-resistant performance. Without a filter, the lens is not dust or water resistant.
Trust Canon to not miss an opportunity to try and sell an accessory, even if their screw-in filters are mediocre quality glass OEM'd by someone else (used to be Tiffen, might still be).
So for these lenses you need a front filter for the lens to be fully rain and dust sealed (I also noted this on the list above for a quick full reference):
EF16-35mm F2.8L USM
EF16-35mm F2.8L II USM
EF17-40mm F4L USM
EF 50mm F1.2L USM
For the sake of completeness, the new EF 16-35mm f/4L IS USM is weather sealed and requires a front filter to complete the sealing.
I really hope so, but I'd be lying if I did not admit that to me the front of the 50L and 85L II are such lookalikes that I almost wrote to Canon, Japan a third time...
What distinguishes the lenses which require a front filter for weather sealing is an inner barrel which moves but the movement is entirely behind the front of the outer barrel to which the filter is attached. The ultrawide zooms (all three 16-35 lenses and the 17-40) all have an inner barrel which moves with zooming, the 50/1.2L has an inner barrel which moves with focusing, and in all cases, that inner barrel movement never extens out as far as the filter threads on the outer barrel. The 85L II has an inner barrel which moves with focusing, but the filter attaches to the front of the inner barrel itself...and in any case, the 85L II is not a weather sealed lens (note the lack of a mount gasket).