I don't think they provide any weatherproofing, or at least I hope they don't. All of mine are deteriorated to the point that all the rubber seals on them are gone, along with the rubberized coating. They are the bane of my car's exterior but I've given up any hope of finding new ones.
1995 Thunderbird LX 4.6- 20% tint, removed air silencer and not much else! (Mustang hubs, Cobra brakes, Vogtland springs, Mark VIII LCAs, Sport shocks and more waiting to be installed)
2013 DIB GT 301A 6-speed- 15% tint, Kooks 1 7/8 mid lengths, off-road x and cat back, Barton 2-post bracket, JPC clutch line, Koni Yellows, Maximum Motorsports C/C plates, BMR K-member, A-arms, UCA & Mount, LCAs and relo brackets, Steeda Ultralites and PHB, Shaftmasters DS, Brembo wheels- Tuned by AED