When I want to vent, I watch Sky News. There's steam coming out of my ears soon enough
It's been a week and nearly 600 views but not one sensible response. Surely someone has tried silicone or has a theory on whether it could be a worthwhile option.
Don't seal it. It's there for a reason
if that's not your thing. Buy a goretex vent screw.
I'm fairly sure, you'll blow the bottom off before the silicone gives way.
Don't take Gesty's advice about the goretex vent. With your faulty O ring there's probably moisture inside the board. That will block the goretex and it won't breath.
Select to expand quotewendell said..
It's been a week and nearly 600 views but not one sensible response. Surely someone has tried silicone or has a theory on whether it could be a worthwhile option.
I was being sensible. OK, a bit brief but I was being serious.
The vent works, its easy, and o-rings can be replaced. I see no reason to screw with it (pun intended)
Select to expand quoteGestalt said..
Sounds like you are talking from experience decrepit
In the early days, I though goretex was a great idea so I did some tests, They work great as long as you have a dry side and a wet side. but if you have 2 wet sides, that's it. It becomes a solid bung, even after the device has had 2 months drying out, it won't vent. I
guess just a small amount of dust or foreign bodies lodge in the weave, and any pressure just forces them in further.
I've repaired a few boards with blown bottoms that used goretex vents. So if you have a board with a goretex vent, after the slightest possibility of moisture getting inside, replace it with a standard bung
Finally an opinion on silicone - if Decrepit's guess is right then it has an advantage over epoxy (plus the above advantages) because it would blow before epoxy (which would never blow). As long as it doesn't blow too easily, but it's unlikely to blow on the water (unless inwards?) so no big problem.
It can be hard getting one's mind around pressures, thus my question. It presumably takes a huge pressure to blow up a board, but how much to rupture a plug of silicone.
Although I didn't want this to turn into a random vent thread, for Mark's benefit, yes I've always been a strict user of vents as they are intended, but sometimes (for us non board builders) we take the easy way. You hear all these stories of people getting away with never undoing the vent, so when you have an old board whose threads get stripped, then when you try to fix it the seat gets damaged so you think I'll just epoxy it closed, the board never gets that hot etc. anyway. Take my board in question ... fun story - when I got it it was 20 years old, then I discovered that JP had failed to drill the bottom of the vent i.e. it'd been closed all those years (and presumably the previous owner had been pointlessly working the screw every session since it stripped on me after about only about ten times) but the board had never suffered from it ... it's in great shape, so rather there's a good chance it benefitted, as in no forgetting to do up the vent, no sand under the o-ring etc.
In that case i would put the screw back in with a skim of wax to seal stripped threads and a skim over top. If you're just saving an old board not worth a new vent install
A solid plug of silicone would not blow in time to save a board. Maybe a wafer thin layer but then all its gotta do is separate from the walls of the hole a little and u have water seeping in
the pressure need to blow the bottom off a board isn't great.
The trouble with open cell foam is that air pressure can find the weakest spot and lift that a little. The expanding air is then trapped there. riding a board over chop, will then "pump" the air, increasing local pressure and enlarging the delam. this process continues until the whole bottom is delamed. Sometimes the weak spot is due to not enough resin, or fatigued foam, due to too much pounding over chop. If the former there will mostly be a clean delam, except where the foam is weaker than the glue. If it's the later, it's a mess inside, the foam gets repeatedly battered every time the board goes over chop, and the can be loose bits of foam just floating around. Very hard to fix.
That explains what happened to my Starboard Carve 90 a decade and a half back. It never had the plug taken out over several years of use.
It went soft a bit under and around the straps with a little bit of water ingress and that got fixed by the Rainman, who was the local go to board mechanic in Sydney.
A couple of years further on and somebody pointed out the 2cm wide crack/ chasm running along the rail for more than a metre that had opened up just as I was about to go out for a sail. I probably should have just carried on sailing that afternoon because the pros deemed it beyond repair, so a bit more water wouldn't have mattered.
How about when someone pulls a board from a hot vehicle and finds it blown up, how much pressure does that take, can anyone do the maths? As in how high could pressure potentially go.
Another option for when a thread is stripped but not wanting to go to the trouble of full replacement; or drilling out for a bigger screw, which would mean finding appropriate screw, drilling, tapping, then you have no seat left so you have to drill again, then make a smooth seat, could be to simply fill the hole with epoxy, then drill and tap for any small screw that you have lying around. Or you could form the thread in one go by using a release agent on the screw. O-ring under the head to seal it fully ... though with this method you'd have to be careful that the hole goes all the way through to the foam. Whatcha reckon?
Select to expand quotewendell said..
How about when someone pulls a board from a hot vehicle and finds it blown up, how much pressure does that take, can anyone do the maths? As in how high could pressure potentially go.
Another option for when a thread is stripped but not wanting to go to the trouble of full replacement; or drilling out for a bigger screw, which would mean finding appropriate screw, drilling, tapping, then you have no seat left so you have to drill again, then make a smooth seat, could be to simply fill the hole with epoxy, then drill and tap for any small screw that you have lying around. Or you could form the thread in one go by using a release agent on the screw. O-ring under the head to seal it fully ... though with this method you'd have to be careful that the hole goes all the way through to the foam. Whatcha reckon?
The pressure change is the ratio of temperatures at the start and finish (in Kelvin). Say your board was stored at 20C in your garage and the vent is closed. You then left in your van on a summer's day when the inside gets up to 50C. The corresponding increase in Kelvin is about 10%. So the increase in pressure in your board is 10% , increased from 1.0 atm to 1.1 atm. (very roughly 14.7 psi to 16.2 psi)
As Decrepit says, it doesn't take much to delam a board.
What you thinking 60C - 70C in the car, coming from what in the shed 25C????
It's not about the temperature it's about the temperature change. and the formula has to work it out from absolute 0 so assuming starting point 25C and car 60C
So 273 + 25 = 298
273 + 60 = 333
That's a change of almost 12%
So say atmosphere is 15psi another 12% makes it 18psi, so that's an increase of 3psi, no where near enough to blow your silicon plug, but enough to lift a poorly attached bottom.
I think it's worse in the hold of a plane, not sure, but Google says at 10km it's down to 4psi. So that would give you a pressure inside the board of 11psi
I'm curious why I didn't follow how you got 18psi - why isn't 112% of 15psi 16.8psi.
Been straining my brain trying to picture this, forgive my ignorance, is the way to understand it by focusing on "pounds per square inch" - so the force on a silicone plug would only be the equivalent of a few hundred grams, therefore not enough to blow it, but the force on say a square section across the width of a board would be (very roughly) 100kg?
Select to expand quotewendell said..
I'm curious why I didn't follow how you got 18psi - why isn't 112% of 15psi 16.8psi.
My bad, I must have used 120
Select to expand quotewendell said..
>>> but the force on say a square section across the width of a board would be (very roughly) 100kg?
I think it's something like that, especially when a small section goes.
With closed cell foam, the pressure is limited to a very small area, but with open cell the whole volume can concentrate on the weakest point. So the same situation applies to air volume, if it starts of at 100l a 12% increase is another 12l that can push a large area of bottom off. Once it starts it's gets more leverage over the lifting face the bigger it gets. until the whole 12l is used.
Select to expand quotewendell said..
It's been a week and nearly 600 views but not one sensible response. Surely someone has tried silicone or has a theory on whether it could be a worthwhile option.
No sensible response because it is not a sensible idea. Now go to the plumbing section of the local hardware store and buy some new o-rings so you can fix this the right way.
Ps I've tried the surf wax way. Total PITA to repair properly after you contaminate the surface like that. Silicone will likely be worse.
Agree get a replacement o ring.
All the technical talk on volume and pressure had me thinking of "Pressure Relief Valves ".
In my work I supply Valves, spares for pressure vessels transporting chemicals - mainly ISO Tanks and road Tankers.
Most are pressure and vacuum that have a poppet with an o ring that seals them with different springs depending on the valve setting.
The lowest I sell is 0.15 bar (less than 3psi) -2kpa vacuum for 1.5" diameter and go up from there and made in stainless with BSP thread.
Obviously way too big and heavy but imagine a small computer printer components assembled to make such a valve for hot climate 
Select to expand quotewendell said..
How about when someone pulls a board from a hot vehicle and finds it blown up, how much pressure does that take, can anyone do the maths? As in how high could pressure potentially go.
Another option for when a thread is stripped but not wanting to go to the trouble of full replacement; or drilling out for a bigger screw, which would mean finding appropriate screw, drilling, tapping, then you have no seat left so you have to drill again, then make a smooth seat, could be to simply fill the hole with epoxy, then drill and tap for any small screw that you have lying around. Or you could form the thread in one go by using a release agent on the screw. O-ring under the head to seal it fully ... though with this method you'd have to be careful that the hole goes all the way through to the foam. Whatcha reckon?
It is not just pressure, its the heat.
Polystyrene, as with any polymer, has no real melting point - it just slowly becomes more elastic and then finally a liquid. So when they say melts at 240C, no it doesn't. It starts to soften at around 80deg and quite noticeable around 100C. I regularly see dark boards placed in the sun or close to the car windscreen (nose on the dashboard) where the styro has completely melted.... up to a 10mm void under board skin.
So if we add styro softening even a tiny bit, to pressure trying to push the board skin off, and also dissimilar thermal expansion of all the different parts of the board well...... bad
As to your last - yes fill with epoxy and use a smaller screw. But I counter with - why? If you have a drill and epoxy sitting there just run a hole saw thru the vent insert and glue a new insert in. $30 and literally 5min.
I have the answer, nature's Goretex - stick a cork in it! Apparently corks let a little air through (helps the wine I hear) ... hang on, bottles go pop when you release them, so .... but still an option.
But why are we still discussing this, jd and 515 solved it with their "sensible" advice to "replace o-ring." Why didn't we think of that! 
Great idea choco! At least in theory. A lot of information could be seen from a balloon, and I think it only takes about 1 psi to inflate one so it would inflate (therefore lowering the pressure inside the board) before the board delaminated. I can see one disadvantage, but it would make for quite a talking point on the beach!
Long thread from using silicone to try and seal a few things (windows, metal window parts, etc) I agree with the following from Google ai:
Look for products labeled "marine grade" or "underwater sealant."
Polyurethane Sealants: These offer superior adhesion underwater compared to standard silicone. They are more flexible and resistant to water pressure.
Ok is this clear for you as requested....
A, don't seal it.
B, don't use silicone it does not stick to fibreglass very well won't stay stuck to 316 stainless and wont last long in the marine environment it won't keep out water and no it won't breathe expand or contract to equalise pressure.
C, Boats use Sikka 290 or 295UV or Fix Tec 15 or Fix Tec 190 HT or 3m 3500 etc etc to stick to fibreglass instead of silicone eg for glazing or deck fittings or to seal skin fittings, or for fixings in to the laminate etc etc...but it's not what you want either.
D, Open the vent after sailing keep it as clean and sand free as you can.
E, once in a while check the 2cent O ring and replace if damaged-you can buy them by the hundreds for next to nothing on line.
F, again don't seal it, its there because without it your board will self destruct slowly or quickly.
Quick google search might help a few bodies here, there are literally thousands of marine sealants made of many
different compounds and they all vary and are good or bad depending on the application.
