# Forum Home Renovation Bathrooms  Neutral cure vs acidic cure silicone

## zacnelson

Hi, I'm curious to know more about what the difference is between neutral cure and acidic cure silicone.  There seem to be so many different applications where one or the other is recommended.  I was advised to use neutral cure over joints before I applied a waterproof membrane in my bathroom; however, in the places where I put the silicone the membrane wouldn't stick to it.  Is this the desired effect?  So, is it true that acidic cure sealant can actually be painted over, whereas neutral cure cannot?  Does normal paint stick to neutral cure sealant where the membrane doesn't stick?  What problems arise when you use an acidic cure sealant below a membrane? 
My waterproof product is Davco K10, which is polyurethane based.  Would it therefore be okay to use a polyurethane based sealant?  And are polyurethane sealants acidic-cure? 
Sorry for all the questions!

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## Dirty Doogie

HI ZAC,  
As I understand it -  
Nuetral cure silicones are used on Metal, painted surfaces, acrylics and "soft" plastics. 
Acid cure silicone are used on fired masonry such as tiles, porecelain, glass (except coated or painted glass), Pvc class 2, timber or timber product and "hard plastics". 
As the name suggests acid cure silicone contains acid which chemically displaces hydroscopic water and oily  molecules trapped in pores on the embedment surfaces.  
Always read manufacturers instructions regarding silicone sealants for plumbing fittings becuase sometimes they specify nuetral sealants where the rule would suggest using acid cure sealants and visa versa. 
I imagine that the areas of silicone you put in your bathroom, are the join lines at wall and floor. If this is where the membrane isnt sticking - that is good as these joins have to have a bond breaker under the waterproofing membrane. 
The K10 membrane is one of the more revered, esteemed and excellent membranes and is hard to damage chemically (with househol chemicals that is!) 
None of the better membranes will stick to silicone. 
Polyurethane sealants and polysulphide sealants (Hybrid silicone/urethane) are another class of sealants that can generally be used where ever silicone is used. The advantage these sealants have over silicone is that they are stronger, can bridge greater distances, and can be used with special primers to really adhere strongly to surfaces. 
Again however anything classed as a polysulphide will have a slightly corrosive effect on delicate surfaces and should be used on the same surfaces as acid cure silicone. 
Acid cure silicone has been known to dissolve pure acrylic and bitumen based waterproof membranes in contact, or at least send them brittle. Yet just yesterday I watched a waterproofer lay an acrylic membrane over acid cure silicone by applying a strip of duct tape over the silicone first. I thought it was dodgy but no one said anything so - - - !!  
Doog

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## Dirty Doogie

Oh Yeah - as a general rule silicones can not be painted over - but i believe Selleys market a paintable silicone. 
Polyurethanes can be overpainted with acrylic.

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## zacnelson

Wow!  Thanks heaps for such a thorough response Doogie.  You have answered all my questions, it all makes sense now.  Regarding the bit about duct tape, I have for a while now been considering using masking tape on at my wall and floor junctions (on top of the silicone) so that the membrane sticks strongly.  It's interesting to see that others do it also.

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