Dow 785+ Grey silicone not curing

[emmaamme]

Has anyone had a problem with Dow 785+ grey silicone not curing?
My tube was only opened in April, doesn't expire till next year & has been stored correctly (cling film over the opening then nozzle screwed on).
I have a tiny bit of silicone on vertical line in shower that's coming away from the wall so rather than remove it all & redo it I was trying to patch it by putting new silicone over it.
I tested the silicone beforehand by putting a small bead on a piece of cardboard & although it took a good few hours it did cure.
I put a thin smear of it on the bit to be patched & after 24 hours it still hadn't cured (still tacky) even though it's a lot less thick than the test piece I did so I've just scraped it off.
Can't understand why as the tube doesn't seem to have gone off.
Anyone have any ideas?

 

[opps]

How did you clean the area before applying the silicone? I normally use isopropyl alcohol.

I suspect that there might have been some kind of gunk under the old silicone which is slowing the curing time.

 

[emmaamme]

I didn't use meths yesterday but have redone it this morning & used meths to clean it beforehand. I'll see if it makes any difference but still tacky at the mo (4 hours later)

 

[emmaamme]

Could it be that if the existing silicone was neutral cure & the new one is acetoxy then the new one wouldn't cure over it?

 

[opps]

Could it be that if the existing silicone was neutral cure & the new one is acetoxy then the new one wouldn't cure over it?

I don't think so, but I am not 100% certain.

I have not experienced that before. That said, silicone over old silicone is never advisable, especially if the old stuff is failing.

 

[emmaamme]

Yeah I know what you mean - it's just such a small bit that it doesn't seem worth redoing the whole line. It's certainly got me puzzled though!

 

[opps]

Photos might help.

It is often the case that the interior face of a shower cubicle should not have silicone on the interior uprights. The part that is screwed to the wall should be siliconed, as should the outer face but many people fail to RFTM.

I am not including you in that list of people. I assume that you "inherited" the enclosure.

 

[emmaamme]

It's a walk in shower & it's the part where the internal walls meet on the right hand side (vertical line which then meets the tray). Just a small section but more annoying than anything else.

 

[emmaamme]

 

[emmaamme]

 

[emmaamme]

I don't think the photos will help much but there was a couple of bits at the side where the silicone had come loose so I was basically trying to go over the gaps with new stuff

 

[opps]

I don't think the photos will help much but there was a couple of bits at the side where the silicone had come loose so I was basically trying to go over the gaps with new stuff

I understand why you want to do that, but it really isn't "best practice". You are basically putting a sticking plaster over an open wound. That said, sometimes you can "get away" with it.

In your last image, it looks like you tried to glue down the old silicone with the new silicone. Regrettably, the old stuff lifted from the tiles for a reason.

Sorry to be pain... but the section is one most prone to getting wet. Given that the old stuff as failed, I would recommend using chisels/etc to remove the old stuff.

Edit- your tiles look as if they are porcelain. Neutral cure or regular 785 should be fine.

 

[emmaamme]

You're right - just trying to put off the inevitable as I hate resealing the shower. Can I get away with just doing the vertical or is it a case of doing the whole shower?

 

[opps]

You're right - just trying to put off the inevitable as I hate resealing the shower. Can I get away with just doing the vertical or is it a case of doing the whole shower?

Balance of probability, you may get away with cutting out sections provided that your silicone seal is sufficiently wide enough and that the old stuff is clean.

You can purchase silicone profiling tools if you need them,

 

[emmaamme]

Thanks for all your help - much appreciated. I did buy a cramer kit which is great however resealing gives me the fear!