Roger Bisby: Filling In with Dow Corning

Roger Bisby: Filling In with Dow Corning


Do you know your neutral curing from your acetoxy silicone? Roger Bisby looks at the key differences.

Most builders, window fitters, kitchen fitters and plumbers use silicone sealants and most of us know the difference between neutral curing and acetoxy silicone.

It’s the smell right? The stuff with acetic acid smells and makes your eyes water whereas the neutral curing silicone releases alcohol which has no smell.

A lot of builders can also tell you that the acetoxy is suitable for kitchens and bathrooms because it sticks well to porcelain and the neutral cure is best for outdoors.

All that is broadly true but there it is slightly more to it. The acidic solvent in acetoxy will attack any material with lime or other alkalis so it is unsuitable for cement, plaster, marble or many types of natural stone because it eats the surface it is supposed to be sticking to and that makes it part company as it cures.

It is also unsuitable for use around metal pipes, especially copper and metal frames because it eats into the surface and causes staining.

For these jobs a neutral curing silicone, such as Dow Corning 791, is best. Not only is it kinder to vulnerable surfaces it is high modulus so it has better stretch.

For items such as window and door frames this matters because they are going to move with the seasons and in the case of aluminium they are going to move a lot on a daily basis as the sun makes its way across the building, particularly the darker frames which absorb more heat.

You can also use 791 for expansion joints. My rule of thumb is that the silicone needs to be at least 10mm deep and you need to back it up with a poly foam filler strip or fibre board, or you can use foam. In order to make best use of the stretch you should stick it to the two adjoining walls but not to the filler.

In the case of foam this is not a problem because it moves easily but a fibre board will grab the silicone a little too well and prevent it moving freely.

You can clean unwanted silicone from surfaces with methylated spirits but once it cures you need to use a silicone remover. Some people use WD40, which is fine, but don’t spray it on the finished job.

It is also very difficult to clean it off effectively if you want to re-apply silicone so the purpose made remover is a better bet if you are renewing.

For more information on Dow Corning click here.

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