What's the problem with silicone on a boat?

[Patrick of M]

Repeatedly over the years I keep hearing "no silicone caulking/sealing on a boat", but I'm wondering why? Is this left over from 30-40 yrs ago when silicone sealants weren't as good as they are now? Sure there are better products 3M 's 4200 and 5200 come to mind. But above the water line in certain circumstances I'm puzzled as to why a top shelf exterior grade silicone isn't an option?
I should add I've been a GC in residential construction for +25 yrs and we use exterior silicone for weather proofing alot.
Thoughts?

[Jebtrois]

Iffy adherence, leaves a layer hard to remove if needs to be reworked or changed later, painting is problematic, most boat stuff does not have the longevity of a house.

[motion30]

Nothing will stick to silicone even silicone itself. The residue is very difficult to remove

[Marathon1150]

Authors like Don Casey specifically recommend marine grade silicone for some applications. For example, Lexan portlights.

There are likely some applications for which it is not the best sealant (e.g. metal to fibreglass).

The only significant reason that I have encountered for not using it where it is useful (like the portlight example above) is that it can be difficult to impossible to get anything else, including silicone, to stick to a surface that has silicone residue, even a very thin film, on it.

I agree with this finding, but, there are ways to remove the silicone. The PO of our boat resealed the opening portlights with silicone and may not have been as careful as necessary to keep the silicone contained. When I installed portlight covers that keep the rain out (mostly) when the portlight is open, the 3M VHB used by the manufacturer would not stay stuck to two of the portlights. Using Goobegone to clean the hull just above the portlight, and new VHB on the cowl, fixed the problem. The portlight manufacturer recommended a product used by car body repair shops but couldn't remember the name, so I tried the above and the replacement VHB tape has held very well for 3 years.

[Don C L]

I have used it, but only for one application. I capped two thru-hulls I don't need and filled them with silicone from the outside. If I ever want to revive them, the silicone plug is easy enough to remove in that case.

[Tupaia]

Use it all the time, hatches, port lights, deck fittings, ABS trim. No you can't paint it so mask up areas before use and use smoothing tool to produce a fine finish on any exposed fillets. Removal of any residual film should you need to move something can be done by scrapping, methylated spirits and a hair dryer. Polysulphide sealants are banned on my boat, they break down crack and turn into a sticky immoveable mess after 10 years. Choose a pure silicone the type used to make glass fish tanks. I have GRP Forespar seacocks that were installed 25 years ago that are good as new. The few that were installed using Sika needed to be resealed after 10 years as did my companion way doors and escape hatches. If you have any Acrylic windows then Dow 795 or GE4000 both silicones used for sticking glass to skyscrapers is the way to go.

[Patrick of M]

Tupaia, yes this is my experience...my boat had various fittings installed using silicone by the previous owner back in the 80s. They are still water tight and secure. This has also been my experience with silicones in roofing etc...stand up to years/decades of wind and rain and when it's time to re roof, it's almost impossible to get off, still strong.

[thinwater]

Lots of good comments. Yes, some of the forum any-silicone is just group think. Another example, is that many hardware suppliers (Rope clutches come to mind) specify ONLY silicon under plastics because butyl and polyurethane degrades the plastic. They even mold it into the underside of the gear!


The trick is using the right silicone remover. After use, you can bond to it at 100% strength. Yes, I have tested this.
https://www.practical-sailor.com/boa...esive-removers


Re-Mov / DSR-5

[s/v Jedi]

It all old wives tales. Every professional uses silicone sealant for marine glazing.

[thinwater]

It is worth adding that there are many grades of silicone. Most of the household bathroom stuff, like most of the household caulks, don't have the durability or adhesive properties of marine and industrial grade products. Very specific silicone products are used for marine glazing, as an example. Dow 795, for example.

[Patrick of M]

Thanks everyone, I had a feeling it wasn't the curse some claim it to be. I'm off to load some bananas aboard.

[MauroM]

A slight sidetrack, but I remember passivating parts of a Volvo diesel in my aluminum sailboat with these phosphoric acid based gel rust removers, only after covering whatever was below it thoroughly to prevent any acid drips on the aluminum bilge (despite the paint).

The same fear came to me when it was time to use a silicone sealant because of the strong vinegar smell (acetic acid) of common silicone sealants. I never used it on aluminum at all.

Here is what the much wiser Google says on silicone types based on their cure chemistry:
........
Acetoxy curing silicones
Also known as acid cure silicones, these sealants release acetic acid when they cure by coming into contact with moisture in the air. The acidic vapor can be corrosive and cause issues like softening other sealants, etching glass, and compatibility problems with concrete. Acetoxy silicones are also known for their fast curing time and vinegar-like smell.

Neutral cure silicones
These sealants release alcohol or another non-acidic substance when they cure, making them a popular choice for sensitive surfaces. They are also more weather resistant than acetoxy silicones, making them a better choice for exterior use. Neutral cure silicones take longer to cure than acetoxy silicones, have less odor, and result in a more flexible seal.

Electrical grade self-leveling silicone
This sealant doesn't contain acetic acid or other corrosive chemicals, making it a good choice for materials that are constantly exposed to the elements, like metal, glass, wood, ceramic, or plastic.
........

In conclusion it looks like the electrical/electronic grade silicone is the best for applications on bare metal. Amazon has a 388 brand that seems to check the boxes see the literature provided.

[Franziska]

If you ever had a paint job ruined because you got a tiny spec of silicone under the orbital sander and did not wash down the boat with copious amounts of silicone remover before painting you will know why professional boat builders hate boats where the stuff was used.



There are only two applications for silicone in my view.
1)Hight temperature silicone for emergency gaskets on an engine or generator.

2)Acid free silicone when installing sta lock or Norseman type fittings.



Apart from that no silicone on this boat. Same applies to galvanised nuts & bolts and A2 stainless nuts & bolts. All a pain.

[s/v Jedi]

The best feature of Dow 795 silicone is the long open time, allowing plenty time to apply to surfaces, position windows etc.

These are for structural glazing, i.e. they put floor to ceiling windows in skyscrapers with this silicone.

Also, the good old household sanitation silicone for bathrooms… even those are not a joke. We once had to demolish a bathroom and it had a vanity sink that was bolted to the wall, with silicone in the seam to prevent water running in between the sink and wall.

After we removed the bolts, we could sit on the sink without it moving at all. No matter what we tried, incl jumping on it, it didn’t move. We had to cut through it all with a steel leader wire.

[Zippydoodah]

Originally Posted by Tupaia

Use it all the time, hatches, port lights, deck fittings, ABS trim. No you can't paint it so mask up areas before use and use smoothing tool to produce a fine finish on any exposed fillets. Removal of any residual film should you need to move something can be done by scrapping, methylated spirits and a hair dryer. Polysulphide sealants are banned on my boat, they break down crack and turn into a sticky immoveable mess after 10 years. Choose a pure silicone the type used to make glass fish tanks. I have GRP Forespar seacocks that were installed 25 years ago that are good as new. The few that were installed using Sika needed to be resealed after 10 years as did my companion way doors and escape hatches. If you have any Acrylic windows then Dow 795 or GE4000 both silicones used for sticking glass to skyscrapers is the way to go.

In my experience, over the years it starts to weather, crack, shrink. So removing it before you replace it is always the best idea. When it was on gelcoat to hide a seam, we had these flat scrapers that made out of Starboard ( which is nearly impervious to IV exposure and doesn't delaminate) that worked 99% of the time.
So when applying, we didn't have time to "tape off " and all that. Running production 25 boats a day. We first clean the area with acetone. Then with the caulk gun we squeezed and filled out spot. Excess..... Foaming window cleaner. Check it out. The cleaner doesn't allow the silicone to stick to your fingers and wipes away clean !! Just run one finger up or down in the direction you traveled with the caulk gun. Whats left in your hand, wile in a towel and DO NOT WIPE the cleaner off anywhere AT ALL!! It will dry and disappear. You'll be good to go. If ya don't believe me..... Just run a bead anywhere and spray it down and wipe like I said. Then you tell me. Peace I'm out....ZIp....