Quote Originally Posted by julex View Post
Make sure your tabs are not cracked at their base. One of mine was. It was fogging up. The cracks allow the water to enter in and cause fogging... I RTVed the tab inside and outside and that cured the problem for about a year now. I chose RTV as at lest it is flexible. You can plastic weld too I guess. If tabs are ok, water enters some other way. Lights should not fog up all by themselves. Vents either allow air freely in, in which case it should equalize the humidity or they don't, in which case you should not have any moist air in there to begin with if the light is sealed right
By design moist air can always enter the light housings as a vented housing intentionally allows that air exchange to handle heat induced expansion/contraction, what you don't want is water entering the housing leaving a standing puddle in the bottom. My housings are (and probably always were) completely sealed, there is no water entering the housing. Rather the condensation is occurring purely as a result of humid air combined with a temperature differential (ie. lens cooled by ambient air and housing/air warmed by the engine). The vents simply aren't able to cycle air fast enough to prevent the fogging. It's exactly the same thing that happens to your bathroom mirror when you have a shower if your fan doesn't move enough CFM.

Quote Originally Posted by julex View Post
Regarding sealant, allroad lights must be different. The sealant is nice, sturdy, silicone it seems - it doesn't get phased by heat.
From what I've read Audi changed the sealant on factory replacements after the production run had stopped, possibly to address heat related failures. Mine came apart with effectively no effort after 10min @ 170F and 170F isn't that far off the temps the lights will be exposed to regularly from the engine bay, especially in stop and go traffic in the middle of summer. Perhaps your lights were replaced at some stage so you have the "upgraded super sealant" ;)


Quote Originally Posted by julex View Post
Anyway, I applied spar urethane last night just to see what it can do and now the light look factory new, wow, it filled these remaining artifacts very nicely. Good stuff. In the absence of mineral spirits I just used medium base lacquer thinner so it dried quicker than just mineral spirits but no complaints. Good stuff.
Spar is a great option for resealing and providing some UV protection. Like you noticed it's also self-leveling and fills in a lot of small imperfections, a lot of guys who use spar to seal lights they're refinishing for clients don't bother with the polishing phase of the refinish and instead just apply it after the 3000grit sanding step. Kinda makes sense given that spar will fill the fine sanding marks and those marks give a little more purchase for it to set onto. I used it on my old A6 lights when I refinished them a few years back, one think I will say is that even fully cured it's much softer than either polycarbonate or the factory UV seal. I scratched the bejesus out of mine using a gas station squeegee to clear caked road salt and crap off the lenses on a skit trip into the mountains. Also, depending on where you live and the UV exposure, spar will breakdown in 6 months to a couple years requiring another round of sanding and resealing. Your mileage will probably be much better given your past experience.

This time around I'm trying Opti-Lens instead as it's supposed to be a permanent and completely optically clear replacement for factory UV. No idea if the claims are true yet, I'll probably apply it this afternoon and see how it goes. These pics are from a thread over on AutoGeek and they're what sold me on Opti-Lens instead of using spar again. 5 months is a bit extreme, but given Hawaii's high UV index this is more of an accelerated wear test for everyone else. The Opti-Lens definitely seems to have stood up much better plus there is a pretty obvious difference in clarity, the spar's yellow colour definitely tints the clear by comparison.

Spar on the Left, OptiLens on the right:




After 5 months of constant exposure to Hawaiian sun and weather: