How to select the right marine sealant

How To Select the Right Marine Sealant

Go to any chandlery or marine store and you’ll be faced with a multitude of marine sealants and caulking materials. In this short article we’ll try and sort them out into some sort of order so you can select the right sealant for each job on your boat.

Essentially, every marine sealant is one of just three products – polysulfide, polyurethane or silicon.

Polysulfide – the “workhorse”

Polysulfide is a synthetic rubber compound and is the “workhorse” of marine sealants. A polysulfide sealant will adhere to joint materials well, but will allow for movement due to temperature or load. It will attack plastics (but not Marelon, epoxy or Delrin). It’s sometimes sold under the trade name “Thiokol”. It can be sanded and painted after it cures (typically 14-21 days). It can be used below the waterline and is good for thru-hull fittings and fixing deck hardware.

Two part polysulfide sealant is used to caulk teak decks.

Polyurethane – the adhesive

Polyurethane is the sealant to use where you’re looking for a permanent seal. It’s an adhesive more than a sealant. Don’t use it if you think you may want to pull the joint apart in the future. Don’t use it on plastics as it will attack polycarbonate, acrylic, ABS and PVC (but not Marelon, epoxy or Delrin ). It can be used below the waterline and is good for hull-deck joints and thru-hull fittings.

Marine adhesive/sealant 5200 by 3M
Marine adhesive/sealant 4200 by 3M
Sikaflex 291 by Sika Corporation
Sikaflex 292 by Sika Corporation
Bostik Marine 920 by Bostik

Silicone – the gasket

Silicone is a gasket material, not a sealant. Its adhesive properties are minimal and should be disregarded. It is great for insulating dissimilar metals such as stainless steel and aluminum and for any above the waterline joint that needs to be dismantled periodically. It is not possible to paint silicone. Up until recently it was the only material you could use to bed plastic (portlights etc.)

Marine silicone rubber sealant by BoatLife
Marine grade silicone sealant by 3M
Bostik 9732 RTV silicone by Bostik
Marine silicone sealant by West Marine

Silicone/polyurethane – the hybrid

There is now a fourth product, a silicon/polyurethane hybrid that combines the adhesive properties of polyurethane with the gasket qualities of silicone. It is now the sealant of choice for sealing plastic components above the waterline.

LifeSeal by BoatLife
Bostik marine 940 FS by Bostik
Elastomeric marine sealant by Sudbury

Selection criteria

So to select the right sealant for your job, you need to answer just three questions

1. Is the seal above or below the waterline?
2. Is one or both of the materials to be sealed plastic?
3. Will I ever want to pull this seal apart in the future?

  • Above the waterline and plastic? Use a silicone/polyurethane hybrid
  • Below the waterline and plastic? You had better be sure that the plastic material is either epoxy, nylon (Marelon), or polyacetal (Delrin), in which case you can use polysulfide or polyurethane.
  • Need a permanent seal above or below the waterline? Use polyurethane
  • Anywhere else – use polysulfide

Unfortunately although we believe all the products mentioned above are of professional marine quality, we cannot make any specific recommendations on which particular sealant brand you should use in any particular situation.

Make sure you read the technical brochures available on the internet for any sealant you intend to use, and pay particular attention to any specific cleaning and priming requirements mentioned in the technical literature.