I've had a few e mails from one or two readers of On Board asking about bedding compounds after I wrote the post about bedding in deck fittings and so on. It seems that there is some conflicting and confusing information floating around so I will try to clear up any misunderstanding.
Walk into almost any marine store and you will find hundreds of different tubes of mastics, sealants and other associated goop. basically there are two different types, polyurethane and polysulphide.
Polyurethane is an adhesive compound of which 3M 5200 is perhaps the best well known and should not be used where there is a chance that you will need to dismantle the joint later. 5200 is used by many manufacturers for hull to deck joints and for this kind of application it is perfect. Polyurethanes also hold up well underwater, will bond to most materials but can attack some plastics but not Marelon so you are OK to use these for bedding in plastic underwater fittings made of this material.
Polysulphides such as Boatlife remain permanently flexible and are my preferred choice for most jobs. They can be used above and below the water line and the joint can be disassembled later if needs be. Polysuplides should not be used for boding polycarbonates such as Lexan or PVC as they will attack them. For this reason most window frames and other components containing plastic are bonded with a silicone product. Polysulphides are available in a variety of colors, I have always found the brown a good match for teak and mahogany but for the best seal with teak use the primer first.
Last but by no means least is the Dolfinite bedding that I have shown above. This is more of an old school product and I like it for applications where I am bedding wood to wood, it has a strong aroma which every time I pop the lid reminds me of old boatyards in much the same way that fresh varnish does. I am getting a bit off topic here but Dolfinite remains flexible for years, will not dry out and will not destroy parts should you ever have the need to take them apart at some later date. As this comes in a can which can be resealed it will not go hard unlike those half used tubes are rather prone to do so for general bedding it could save some you some money in the long run.