How do you clean the contaminant residue left behind to get a good seal when rebedding?
You will need to sand it after removing as much as you can by hand. Tape off the areas you don't want sanded and then sand the contaminated area. Certain plastics can have the plasticizers leeched out of them over time from products like polysulfides. This leeching makes the brittle and more prone to failure. In these situations I would recommend Dow 795 as the Silicone of choice..
As Dog stated silicone has very, very few good uses on a boat. Certain plastics require it because they can be attacked by Polysulfides, Polyurethanes or Polyethers.
The problem with Silicone is that it nearly permanently contaminates a surface. Even when you think it is gone and you've rubbed it away it is still there. Simply use a spray bottle, like for ironing, and mist the surface it the water beads the surface is contaminated.
Silicone was and is designed to resist attack by most all chemicals. None of the commercially, body shop strength, removers are really all that safe for fiberglass and even they don't work all that well even on metal.
Once you've used silicone it will be very tough to get anything else will stick to where you had the silicone.
There are also MANY grades of silicone and it goes much deeper than just marine or bath tub. Acrylic hatches should only ever be sealed with Dow 795 or GE SG4000 as these are "structural grade" Silicone products. Bath tub silicone is not even rated as such and should not be used to seal ports or hatches as it is just not safe or prudent behavior.
If you are going to choose a Boat Life product stay away from the combo Life Seal product as you WILL get silicone contamination and choose Life Calk for general bedding uses unless certain plastics like Polycarbonate or Acrylic..
For those of us who have had to atually remove silicone contamination, for paint work, I can asure you the ONLY way to 100% get rid of Silicone contamination of gelcoat is to sand it off.. Just because you can't see it does not mean it is not there, trust me!!