# Which silicon to use



## emlclcy (14 Apr 2022)

Installing a upvc window into a brick opening, which exterior silicon would you use to weather seal it? There are so many at screefix!


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## Ollie78 (14 Apr 2022)

Normally silfix or Dow corning are fine, something that actually advertised as a frame sealant. 
You might want to use a hybrid polymer especially if painting over it.
My tip is buy some proper rough surface masking tape for the brick side and use it.

Ollie


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## Yorkieguy (14 Apr 2022)

I don't claim any expertise in this area, but I believe that the sealant for external use on upvc frames should certainly be silicone - not one of the many acrylic type sealants. The reason for this is that upvc frames expand and contract with temperature variations to a greater extent than do wooden frames, and silicone remains flexible to cope with that expansion and contraction, whereas acrylic sealants harden over time and eventual crack.

Dow Corning have enjoyed a good reputation for all types of sealants for decades, and the one at the Screwfix link below seems to get excellent reviews. A bit dearer than other brands, but why try to save a fiver, considering the overall cost of the window?

Dow 796 uPVC Silicone Sealant Brilliant White 310ml

Hope that helps.


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## Spectric (14 Apr 2022)

Hybrid sealants like OB1 are great for many jobs where silicon used to be used, the only place I tend to use silicon these days is sanitary ware and showers, here DOW 785+ is ideal with Cramer profiling tools.


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## mikej460 (14 Apr 2022)

I've just used this around an external door and it is excellent stuff Sika Sikaflex EBT+ All-Weather Sealant White 300ml | Weatherproof Sealants | Screwfix.com 

Having said that I've also used this which is also very good No Nonsense Exterior Frame Sealant White 310ml | Glazing & Frame Sealants | Screwfix.com


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## lexi (15 Apr 2022)

It should be a low modulous silicone that you use. The low mod and high mod have different BS numbers.


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## Bingy man (15 Apr 2022)

I have used Dow Corning most of my working life and I can agree with the quality as long as it’s frame sealant, it used to retail at £5.50 to £6.50 but the price now can be £9.50 to £11.00 . if they produce a frame sealant it will be worth the price . Just cut the nozzle to width of gap and cut the sealed tube end square. Read the spec before you buy as it needs to allow for expansion/ contraction as per other posts .


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## baldkev (15 Apr 2022)

Low modulus for porus surfaces, high modulus for shiny surfaces ( tiles etc )



Spectric said:


> Hybrid sealants like OB1 are great for many jobs where silicon used to be used, the only place I tend to use silicon these days is sanitary ware and showers, here DOW 785+ is ideal with Cramer profiling tools.



Hybrid sealants are a compromise... often overpaintable, which is good, but they simply dont profile as easily as silicone.
Paying for a high quality silicone is 100% worth it. Both in logevity and workability.
Water with a touch of fairly liquid in a spray bottle works wonders 
As a compromise, sikaflex is great. Not as easy to profile as silicone, but a great product, long lasting and flexible


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## RobinBHM (15 Apr 2022)

any quality low modulus silicone is fine.

try to keep the bead small as silicone weathers quickly externally.


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## Spectric (16 Apr 2022)

baldkev said:


> Water with a touch of fairly liquid in a spray bottle works wonders


Something you should never do, don't use liquid and don't use a finger if you want great results.


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## baldkev (16 Apr 2022)

Spectric said:


> Something you should never do, don't use liquid and don't use a finger if you want great results.



All the pro's do... over the years weve had numerous sealant companies in and they all used a bit of fairy.... i do too, and it is noticeably better to profile cleanly


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## Spectric (16 Apr 2022)

Yes it makes it look better and easier to apply but at the risk of adhesion, silicon does not like sticking to wet or greasy surfaces. The Pro's may do it because it gets the job done quicker and easier but it is not there shower or bathroom.


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## ian33a (16 Apr 2022)

Spectric said:


> Something you should never do, don't use liquid and don't use a finger if you want great results.




Well, that's an interesting video and, to be honest, it makes a lot of sense. Thanks for sharing it.

During my whole life I've cut the nozzle at 45 degrees, applied the sealant and then swished my finger in a jam jar containing water and a very small bit of washing up liquid and _human profiled_ the joint. I've also used masking tape to get the bead linear on occasions, then used my swished finger to remove the ridge. 

I shall invest in one of those tools going forward.


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## pe2dave (17 Apr 2022)

ian33a said:


> Well, that's an interesting video and, to be honest, it makes a lot of sense. Thanks for sharing it.
> 
> During my whole life I've cut the nozzle at 45 degrees, applied the sealant and then swished my finger in a jam jar containing water and a very small bit of washing up liquid and _human profiled_ the joint. I've also used masking tape to get the bead linear on occasions, then used my swished finger to remove the ridge.
> 
> I shall invest in one of those tools going forward.


After watching that I did, and did not regret it. Makes for a lovely finish - and no sticky fingers!


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## baldkev (17 Apr 2022)

Spectric said:


> Yes it makes it look better and easier to apply but at the risk of adhesion, silicon does not like sticking to wet or greasy surfaces. The Pro's may do it because it gets the job done quicker and easier but it is not there shower or bathroom.



Yes, thats why it works better, and thats why the bead is incredibly important. Most guys cut the nozzle to a taper and angle it into the work area at about 45°.
The ideal bead meets both edges of the work area and the nozzle gives a first attempt at profile as it passes over.... this basically pushes the silicone into the area being sealed, sealing both edges. If there are any unsealed areas, you can run a damp finger over that spot to make it seal. You then ligthly spray the water and fairy mix over the silicone and profile with a fugi ( or the professional guys just have tapered wedges of wood at whatever width because they'd kill fugi tools for a passtime )


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## mikej460 (17 Apr 2022)

I've always marvelled at how builders and other professionals can apply perfect silicon runs without using a finger or any smoothing tools. I've used hybrid sealant/adhesive and it is very useful but not if you want perfectly smooth results. However, I've found the clear hybrid versions do smooth out better than white or black, and over painting hides a multitude of sins!


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## baldkev (18 Apr 2022)

mikej460 said:


> I've always marvelled at how builders and other professionals can apply perfect silicon runs without using a finger or any smoothing tools



You can get very good results off a nozzle, but theres often a slight imperfection as you reapply pressure on the caulk gun handle on a longer run, and where there are internal corners ( best thing is to work from the corner out to limit buildup )
Whichever way you do it, you need to avoid the thin film of silicone that gets left behind of not cleaned back properly. After a few months it starts to show and after a couple of years it looks rubbish, usually dirty and peeling


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## Spectric (18 Apr 2022)

If there are any welders around then it is easy to compare the silicon fillet to a welding fillet where you want even legs and a smooth finish, for me I can lay a nice welding fillet in gas or TIG much better than a silicon one and get in less of a mess!


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## thetyreman (19 Apr 2022)

can also recommend dow corning, I have tried some of the cheaper brands and frankly they are rubbish, not worth saving money on, also get some backer rod to fill in the gaps and you'll save money on sealant, it fills in most of the gap and can also expand and contract without causing cracks in the sealant later on and will remain airtight.


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## Majimoto (20 Apr 2022)

Where does Tec 7 come in the hierarchy of silicones?


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## pe2dave (20 Apr 2022)

Seventh place?
;-) Sorry....


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## Shaun92 (22 Apr 2022)

I usually use this Dowsil because it is a reasonable price compared to others and I’ve never had any revisits due to it peeling off. Just make sure you have a half decent gun, the nozzle cut to the right size and I usually use a squirty hand soap to flatten off the beads with my finger. Tooling it seems harder when your running against brickwork.



https://amzn.to/3MtnB85


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## Sachakins (22 Apr 2022)

Silka SilkaFlex


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## Housey210 (1 May 2022)

PU mastic, far better.


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## Housey210 (11 May 2022)

moved away from silicone years ago! PU,


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