Kwazii

So my hatch glass is completely separated along the top edge. My plan is to remove the glass and reseal the whole thing properly using Sitka sealant. The only issue is that the previous-previous owner "caulked" it. I don't know if it was water-based caulk, or silicone but I'm just going to assume silicone. Is it possible to completely remove the silicone and get proper primer adhesion, or should I play it safe and get a new hatch? I don't want to waste time doing this if silicone will still be in the glass pores and prevent the primer from sticking.

931guru

Glass doesn't have pores.

Remove the caulk using a razor blade at a shallow angle, then scrub the surface with an abrasive pad soaked in mineral spirits to get rid of any silicone residue.

jhowell371

Any place where even a trace of silicone caulk remains nothing will stick so clean it well, usually sanding is required. Way back when I was a an aircraft mechanic I used to see aircraft owners apply layer after layer of silicone caulk one on top of another in a futile effort to stop a window or windshield leak. Sometimes the results were comical with sealant smeared for an inch or two across the window and fuselage. While recently junk yard scrounging parts for the wife's VW Cabriolet I came across a pitiful VW ragtop with a 4 inch wide band of silicone caulk from the top of the windshield to back over the convertible top in an attempt at stop water.

eshane

38 years as a Missile design engineer, materials engineering subject matter expert, with Lockheed Martin Aerospace. Once silicone is applied to any surface, it can never be entirely removed. Silicone has a completely different atomic structure from carbon-based organics and does not react to any petroleum-based solvents like typical carbon-based organic materials (epoxies, polyurethanes, elastomers, et. al.). Once silicone comes into contact with a surface, nothing will ever stick permanently to it again. Clean it to a fair-thee-well, sand it, grit-blast it, acid-etch it, what-ever. Silicone migrates terribly, forms a mono-molecular layer (single atoms thick) that is detectable only with laboratory equipment. For permanent silicone bonding, silicone primers that form this thin layer are used, because silicone atoms don't attach to carbon-based atoms very well.
No silicone-based products are ever allowed on the production floor, or even allowed in the manufacturing building, where bonding or painting is performed. Out in the real world, exposed to atmospheric conditions (humidity, rain, melting snow, temperature extremes, sunlight, etc.) it will eventually debond. Ask me how I know.........

Dan Martinic

May I ask: to keep a flapping original windshield rubber surround trim down, I smeared silicone into its track and kinda glued the surrounded down. When time comes to replace my windshield (soon), will I have an issue? I assume I can buy new surround trim

eshane

I'm not an expert on 944 windshield installation, but looking at mine, I don't believe the rubber surround is part of the actual windshield bonding operation. It appears to just "grab" the windshield edge in a groove. The actual bonding agent adheres to the windshield frame structure. I'd check the price of a new rubber surround, it may be just a minor cost in relation to installing a new windshield and may well be the smarter thing to do. I'm assuming the windshield is being replaced due to damage, not debonding from the structure. As an example, put some silicone on a surface, smear it, then do everything you can to remove the residue. When it looks really good and clean, I mean really really clean, take a piece of adhesive tape, strongest you can find, and stick it to the supposedly clean surface, remove the tape and then fold it over on itself, sticky-side to sticky-side. You will be able to pull it apart easily, both side have been contaminated with the silicone residue that remained after cleaning. Nasty stuff silicone. Works really fantastically in some applications, but woe to the assembler that has to rework something that was bonded with silicone, unless is is reusing silicone to rebond. May look sound, but the bond really has no adhesive strength.

SecaBlue

Back on the original post question... out of curiosity, how do you know it is silicon?... because it is clear? I have a feeling that my rear hatch has had an attempt at a re-seal using something black (I can tell because it appears quite messy in places), and is also separating in places although no leaks yet... no idea what they used though.

SeaCay

It's kinda scary the amount of info/knowledge/experience Rennlist has a collective. Thanks for your input.

SeaCay

Which SIKA sealant/adhesive is correct?

Kwazii

Maybe microscopic "inconsistencies" is a better descriptor... *shrug*

Thank you, that's basically what I assumed, but didn't have the expertise to back it up

PS: sounds like you get to work on some cool stuff!

I don't know, so I'm just assuming the worst-case scenario. If it's black, I'd guess that's some kind of automotive silicone, like RTV/gasket maker.

I haven't gotten that far yet. I've just been doing a lot of reading about people resealing their hatches and the consensus seems to be that the 3M stuff fails, and Sika is much better because of the 2-part primer. Example. I'm going to check out a local windshield/glass place to see what they use and buy from them if possible.


Sounds like my best bet is to keep an eye out for a nicer hatch. My rubber spoiler is badly UV-damaged so a new hatch would kill both of those birds. Thanks, all

SecaBlue

But Sika also comes in black (have used it a lot in yacht decks)... so don't judge it by the colour.

I'm wondering if there is any kind of 'test' you can do to determine if it is silicone before you get too excited about replacing the hatch? I can cut a small amount off mine to check if there is a way

Kwazii

I contacted Sika for a product recommendation since they offer so many, and this is what they had to say: