[Masticator]

I do not recommend using anything other than butyl to seal the lens back to the housing. Silicone and other chemical glues have the possibility to emit strong fumes inside the light and possibly stain or leave a film on the internals.

A method I've learned from working at LightWerkz is to heat the light up 235F for 15-20 min (after having the lens off) and pushing down the original sealant with a flat head screwdriver while it is still hot. When we open up a light with a good amount of high quality butyl sealant, we try to minimize the amount of sealant that we remove in this process by keeping the flat head as horizontal as possible; it is more like smoothing it out uniformly, rather than scooping it out.

If a light's sealant is not up to our standard, whether it is just crap quality aftermarket sealant or just very old/dirty/dry factory butyl, we do indeed scoop it out as much as we can. It may take several rounds in the oven but eventually you will end up with a light housing fairly free of butyl. It is tough to get 100% of it out, but another tip for removing butyl reside is to gather some up into a ball and simply press it against the butyl that you would like to remove. Note that this doesn't work too well on trying to remove a large amount at once and that there is a "perfect" temperature where it comes off cleanly.

But if you do indeed feel that the original butyl sealant is compromised, try the scooping method above.

The best product to use here is Morimoto RetroRubber. OEM quality butyl rolled/stretched out into a manageable rope.

It's available in black or gray and in its unstretched state as it arrives, I find it to be the perfect size for filling most headlight channels nicely. It is a bit easier to work with if you warm it up for maybe 2-4 minutes at 235F (just toss the whole box in the oven.) Any longer than that and it can become unwieldy, as it stretches easily when hot.

And as for the windshield butyl that 2.5x_sleeper had mentioned, I also do not reccomend using that. It will work in a pinch, but it is overly sticky and leaves disgusting residue on anything it touches,

 

[Masticator]

I have opened and closed many, many, many headlights using the 235F at 15-20 mins method without any problems. Subaru lights are always good quality (anything from Ichikoh/Koito is imo) and hold up just fine to the oven heat.

We try to avoid using a temperature higher than 250F as things can get a bit hairy, but some lights you just need the extra heat/time to coax them open.

Just keep in mind ever oven is different, so maybe use a secondary thermometer and keep an eye on the light every 5 min or so.

Some lights do not like the heat of the oven, particularly German and American OEM lights and can sometimes incur damage to the lens. But rest assured, like I mentioned before, Subie lights are quite resistant to oven damage. We see PLENTY of Impreza lights come back multiple times, not due to problems, but because the owner wants additional modifications done, to the same light. They can take the opening and closing process just fine, although I feel that the butyl can get a bit "tired" after over 3 or 4 times, but that is why we add extra when needed.

Here is a crude explanation as to why headlights can condense/hold water I have made on a different thread. Maybe ignore what I said about the sticky sealant, since you say you have butyl. I have not seen an SG light using butyl yet, but I have only opened maybe 3 or 4 pairs. OEM suppliers always change stuff as I have noticed.

The sealant on the OEM SG lights is not the best. It is a sticky, stretchy adhesive which does not harden. This same sealant is currently used on the 2015+ WRX. It is different from the typical butyl rubber which can be heated in order to open the lights, and also cooled to a very hard (yet still slightly malleable) state for closing up the lights.

Reasons for condensation are twofold:

-The actual seal may be compromised, meaning moist air or straight up water can enter the light. Modern car headlights are not 100% water/air proof so it is actually normal for humid air to enter/accumulate inside the light.

Additionally, the rubber seals/caps for the bulbs can damaged allowing water/moist air in.

-Headlights are meant to "breathe" so to speak. They can have vent tubes/holes, goretex patches, silica packs, or a combination of those. All of which assist in the alleviation to evacuate the moist air from the interior of a headlight.

Inadequate venting from the factory, clogged/damaged vents, or even damage to the lens/housing itself lend themselves to water droplets forming inside.

Quick and dirty (which i do not really recommend) fixes include drilling into the rear housing, removing the dust caps, and adding silica packs into the light. These fixes on their own *may* work for minor cases, but they are merely a band aid so to speak.

The "correct" fixes are either replacement of the light or baking them open to clean/inspect the seal and vents, and resealing with new butyl rubber sealant. Adding vent holes and sticking goretex patches said holes, and also adding silica packs for extra insurance.

Here is a video of LightWerkz opening and resealing a headlight for your informative purposes

 

[Masticator]

@GT Premi

Butyl does indeed become quite hard when it cools to room temperature. It is still a bit pliable, but it should not be super flexible like the stretchy sealant 15+ WRX use.

Here's a vid of said WRX sealant https://www.facebook.com/lightwerkz/videos/1069935883029927/

Maybe check the lens/housing for markings/stamps. If a Japanese car's lights say NAL (logo looks like NFL, Koito, Ichikoh, Stanley, or even the actual manufacturer of the vehicle, then they are likely OEM lights.

And as for the lenses being glass; the only Forester to have glass headlights was the 98-00 SF. After 00, they went to plastic after the face lift. They have been plastic on every iteration of the Forester since then.

To answer your question about waiting before reinstalling, if you seal them up properly, they should be good to go instantly. But for good measure maybe just let them cool down.