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John
Yes it is a complex issue. I did some work with the Australian Sika company a few years ago and became convinced about the bonding properties but as you and others infer, there are various grades and that is why I suggested the website for further information.
The problems with silicone go beyond what have already been mentioned - one of the main issues is the inability to remove traces of it wherever it gets - and it will get everywhere, no matter how careful - this makes a nighmare for painting. Conversely, uncured polyurethane can be easily wiped off with a paper towel or rag soaked in turps and can be painted over.
The second problem is longevity. Silocon seems to work fine, but over time the bond lets go. That does not happen with polyurethane, especially if the substrate is treated with the appropriate primer. Homeowners have found this out the hard way in shower stalls and other areas wher the plumber has used the cheap method
You have now exhausted my knowledge of the subject, but so far, it has worked as described
Cheers
Alan [/i]
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