[brian eiland]

....this appeared in an online publication of SailWorld...too good to pass up, Brian


Many of the yukkiest, most pointless, timewasting activities in boat building and repair are all down to the misuse of silicon sealants. Generally if there is a leak some handyman type will try and fix it with this awful stuff. It is absolutely fine in bathrooms and houses but it just does not belong on boats. Why?

You can't paint it.

You can't sand it.

It doesn't stick well enough to be structural or to stop boat type leaks in its own right

It doesn't stick poorly enough to be easily removed.

When the thinners from paints hit it they spread its unpaintability to nearby areas which it hasn't contacted directly. YOu can the wash and sand to try and get rid of the residue and it just spreads it over the whole area so that the paint bubbles in an intermittent sort of way.

The only place it works is underneath fittings that are bolted or screwed to the boat - but ONLY on boats that will never be painted - even fibreglass boats may be painted one day - so what boats could they be?


Sikaflex, yes, yes, yes. - .. .
Use a polyurethane sealer like sikaflex. It can be sanded, painted and it doesn't really cost much more. It also seals gaps in a structural way - so if the bits move relative to each other it will still keep on working.

In fact you can glue a whole boat together with sikaflex - not that I'd recommend it - epoxy does a better job in most cases - but there are places...

Disclaimer - if I sound bitter it's because of bitter experience. I worked as a professional boat painter and varnisher over several years - silicone sealant was the #1 reason for having to redo work that we thought was finished.

About the author:
Michael Storer supplies Wooden and Plywood Boat Plans for Amateur Boatbuilders. They specialise in Light, Elegant, High Performance, and Simple Construction.

[Neil Rooney]

Sounds like Andy Rooney from 60 Minutes!

[Capt. Joe]

Oh Yes, I have nagged on about the use of silicones in newbuilds and in repairs for too long. Nice to see someone else gettin the drift;

Silicone has no place aboard a boat

... except maybe to use a drop to secure your favorite nick-nacks.

And, to think there are actually products on chandlery shelves that claim to be "Marine Silicone".....

[sbmar]

Gentlemen:

Although, in general, I do agree with what has been posted (Acetoxy based silicones—the type that is evidenced by the vinegar like odor during the cure) , you are leaving out what I call, a hybrid form of silicones that do have some wonderful properties and will out perform any of the polyurethanes in some very select applications.

These silicone adhesives or caulking compounds (Alkoxy or Oxime based silicones), are 100 % non-corrosive and use moisture as a cure mechanism (similar to many polyurethanes and polysulphides), but have not made its way into the general use marine market for various reasons. I consider them an undiscovered but vital tool for our work that can be used in many applications such as bonding stainless steel to FRP substrates without fasteners, bonding glass and acrylic windows to a FRP substrate without fasteners, sealing aluminum and SS fabrications when you need a superior bond but want a much softer durometer sealant ( movement between two pieces is inevitable and but a seal must be maintained), and certain other application where heat may be a factor ( polyurethanes soften quickly above 140F). Like the common silicone, paint does not adhere well, BUT they many clean up very nicely with common mineral spirits when uncured. Many can also be used (and are preferred) in electrical potting & sealing applications application when epoxies are not applicable.

Another important point that needs to understood when using any of the common polyurethanes ( 3M 5200, Silkaflex , etc) is that although they are quite strong for “gluing” things together (FRP and wood for example) , in many cases, relying on the “glue joint” for many applications, is only skin deep.. What I am referring to is whether the bond breaks "cohesively" or "adhesively"—big difference and can be very important--- a typical example would be 3M 5200 squirted onto a piece of smooth or polished stainless steel. Left to cure, it will then peel off like a band-aid.. Its adhesive properties are minimum compared to its cohesiveness on many substrates.. On the other hand, a product like GE 4000 Ultra glaze will not de-bond, but only let go cohesively-- the bond is stronger than the product itself. Kinda like a good wood glue—The wood breaks before the bond fails.

Some common applications where you might consider the use of a silicone product will be the bonding of acrylics & glass, SS, and aluminum where mechanical fasteners cannot be used. We also use Ultra-glaze 4000 exclusively as a sealing agent ( like a pipe dope) for the assembly of larger brass piping used in salt or fresh water applications -- in our work we cannot afford even the slightest weep,seep or "sweat" from a typical fitting or hose connection.

Again, I am not here to disagree with “Silicone has no place aboard a boat”, only to expand the knowledge on what the right type of silicone can offer.


Just a “Marine Nut Lookin’ In”

Tony

[brian eiland]

Thanks Tony, that was tremendously informative