Most people are familiar with silicone caulk, since it is the most commonly used type of caulk in normal household repairs. However, while certain Silicone-based bedding compounds are excellent for bedding ports, most of the time, silicone caulks really serves no useful purpose on a boat beyond bedding ports and covering the ends of cotter pins.
In most cases, on a boat, you want a sealant that will adhere to the two surfaces being joined—whether it is a through-hull and the hull or a cleat and the foredeck. The sealant should have a fair bit of elasticity so that it can stretch when the hardware moves or shifts under load without detaching from either side and breaking the seal.
For deck hardware and through-hulls, countersinking the fastener holes is probably one of the best things you can do, since it gives the sealant a natural place to form an o-ring like seal.
There are four major classes of bedding compounds/sealants used on a boat.
* Polyurethane-based sealants like 3M 4200 and 5200, SikaFlex 291, 292, 295, 296
* Polysulfide-based sealants like LifeCaulk and 3M 101
* Silicone-based sealants, like Dow 295
* Butyl Rubber Glazing tape—this is not butyl rubber caulk
Each of these sealants has specific pros and cons.
Polyurethane-based sealants are basically adhesives with sealant properties. They are often very permanent and have very strong adhesion strength, and can be used both above and below the waterline.
3M 5200, a polyurethane sealant commonly found in marine chandleries, is basically for all intents and purposes a permanent adhesive and should not be used on boats for the most part. 3M 5200 has a bonding strength so high that it can often cause delamination or damage the gelcoat when you try to remove hardware bedded with it.
However, polyurethane sealants have some of the best materials compatibility, so the less aggressive ones, like 3M 4200 are very useful. SikaFlex 291 and 292 are probably better choices, but usually more difficult to find. For bedding ports, Sika 295 or 296 can be used in place of Dow's 795 Silicone.
Be aware that using a polyurethane sealant can make removing hardware much more difficult than using other sealants. There is a solvent, called DeBond 2000, which can be used to weaken the bond if you need to remove hardware that was bedded with 5200. One other issue with the polyurethane sealants is that they have a relatively short shelf-life, once opened. This is due to their being moisture curing compounds, and once exposed to the moist sea air... they start to cure...and you eventually end-up with a solid tube of cured sealant.
In most cases, on a boat, you want a sealant that will adhere to the two surfaces being joined—whether it is a through-hull and the hull or a cleat and the foredeck. The sealant should have a fair bit of elasticity so that it can stretch when the hardware moves or shifts under load without detaching from either side and breaking the seal.
For deck hardware and through-hulls, countersinking the fastener holes is probably one of the best things you can do, since it gives the sealant a natural place to form an o-ring like seal.
There are four major classes of bedding compounds/sealants used on a boat.
* Polyurethane-based sealants like 3M 4200 and 5200, SikaFlex 291, 292, 295, 296
* Polysulfide-based sealants like LifeCaulk and 3M 101
* Silicone-based sealants, like Dow 295
* Butyl Rubber Glazing tape—this is not butyl rubber caulk
Each of these sealants has specific pros and cons.
Polyurethane-based sealants are basically adhesives with sealant properties. They are often very permanent and have very strong adhesion strength, and can be used both above and below the waterline.
3M 5200, a polyurethane sealant commonly found in marine chandleries, is basically for all intents and purposes a permanent adhesive and should not be used on boats for the most part. 3M 5200 has a bonding strength so high that it can often cause delamination or damage the gelcoat when you try to remove hardware bedded with it.
However, polyurethane sealants have some of the best materials compatibility, so the less aggressive ones, like 3M 4200 are very useful. SikaFlex 291 and 292 are probably better choices, but usually more difficult to find. For bedding ports, Sika 295 or 296 can be used in place of Dow's 795 Silicone.
Be aware that using a polyurethane sealant can make removing hardware much more difficult than using other sealants. There is a solvent, called DeBond 2000, which can be used to weaken the bond if you need to remove hardware that was bedded with 5200. One other issue with the polyurethane sealants is that they have a relatively short shelf-life, once opened. This is due to their being moisture curing compounds, and once exposed to the moist sea air... they start to cure...and you eventually end-up with a solid tube of cured sealant.