New silicone will not adhere to old silicone, so therefore it must be completely removed.
Google remove or dissolve silicone, there are lots of products that will help remove any residue. Some can even be had at Lowe's or Home Depot.
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Pete
"I never make mistakes...
I thought I did once, but I was wrong"
Current Tank Info: In the process - http://reefcentral.com/forums/showthread.php?t=2661614
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