September 30, 2010

Sticking to the basics

ONCE IN A WHILE I get the urge to say something useful, instead of waffling on about sailing in general. This is one of those occasions. I am going to talk about glue.

Too many sailors, like Old Wotsisname with the concrete boat, think epoxy resin is the be-all and end-all of glues. They use it for everything. Well, they are misguided.

Most glues used in boat work fall into one of four categories:

(1) Rubber, natural and synthetic. This includes contact adhesives.

(2) Melamine-urea types. These are water-resistant glues suitable for amateur use, including Weldwood plastic resin glue, Casco urea-formaldehyde, and Aerolite resin glues.

(3) Epoxies. Well-known epoxy resin in its many forms is excellent for much marine work, including laminating new fiberglass to GRP hulls. It fills gaps and forms an effective sealer coat. But most epoxy is not waterproof. Should I repeat that, or were you listening the first time? It’s water-resistant and may be adversely affected by salt water and sunlight.

(4) Resorcinols. The best wood-to-wood glue available for marine use is resorcinol. It’s fully waterproof and not affected by sunlight. It comes in two forms. One needs temperatures of 70 degrees F (21 degrees C) or higher to cure. The other, an imported version made by Ciba-Geigy (Aerodux or Cascophen) will cure in temperatures as low as 50 degrees F (10 degrees C). The latest versions are also gap-filling. The American product is made by Weldwood and is available at the best marine hardware stores.

Some modern bedding compounds are almost as adhesive as glue and might be considered glues in their own right. I’m thinking of 3M 5200 and its peers.

Polyurethane sealants form strong, long-lasting adhesives with gap-filling ability. They do not require clamping and are easier to clean up than epoxy is.

You probably know already, but may have forgotten, that plastic materials used on boats fall into two groups:

(1) Thermosetting plastics such as polyester resin, Formica, and melamine.

(2) Thermoplastics such as PVC, acrylic glass substitutes, nylon, and polypropylene.

When you heat them, thermoplastics soften and melt. Thermosetting plastics do not. They become permanently hard and unmoldable after their initial expose to heat.

So watch out, because some glues, sealants, and bedding compounds will melt certain plastics. Always read the manufacturers’ fine print.

Today’s Thought
Because something is possibly possible,
It doesn’t follow that it is necessarily necessary.
There are sea-going Folk and then there are
Those shore Bastards.
— Thomas E. Colvin, Naval Architect

Boaters’ Rules of Thumb, #101
A hook is likely to be the weakest part of any fitting. The old rule of thumb to find the working load of a hook in tons is diameter squared divided by 2, where diameter is in inches of the metal at the back of the hook.
For a D shackle, the working load is diameter squared times 3.
Bow shackle: Diameter squared times 2.5
Ring bolt: Diameter squared times 2.
Eye bolt: diameter squared times 5.

The mall security man came across a little boy in tears. “I’ve lost my dad,” the boy said.
“What’s he like, young feller?”
“Beer and blondes.”

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)


oztayls said...

"Most epoxies are not waterproof." Yes, you are absolutely correct! I'll just add the reason why this is so, if I may?

A number of older technology epoxies contain solvents, and the evaporation of these solvents through the epoxy leaves microscopic tubes through which water (and air) molecules can flow. Just about any glue or paint that cures by the evaporation of a solvent won't be waterproof. This is also why steel will rust underneath paint.

There are now more modern epoxy glues and epoxy paints that are "high solids". They exclude air and moisture very well, and are waterproof, and they are the beez kneez when it comes to glues, sealing boats and stopping rust in its tracks. There are even "one part" epoxy paints now that provide a complete seal against moisture and air.

Yes, not all epoxies are equal, but the good ones rock when it comes to boats.

However, most epoxies don't have much resistance to UV, so they need a UV resistant coating to protect them.

As you may deduce, I'm an epoxy convert and when it comes to gluing or sealing boats, nothing else comes close in performance or will last as long. Viagra for boats it is.

Nikolay said...

Can anyone enlighten me as to where Tightbond 3 fits into this tale?

oztayls said...

Hi Nikolay

Good question. The problem is that a lot of people, mainly amateur boat builders, are seduced by some of the claims that Titebond makes, and it finds its way into boats. Unfortunately, they don't specifically say that it's unsuitable for marine use, and leave that for users to decide for themselves.

Titebond 3 is not an epoxy resin. It is a water based aliphatic PVA resin. Although highly water resistant for outdoor applications, it is not suitable for marine use.

It is not gap filling like epoxy, and cannot be used for filleting, waterproofing or taping joints. It also tends to creep over time, which makes joints underneath paint unsightly and therefore it is also unsuitable for laminating. Other glues and marine finishes will not adhere to PVA's.

However, it's a very good and convenient glue for general outdoor use where joints are well made and tight fitting. I guess that rules out a lot of us amateur boat builders :)

Nikolay said...

Does anyone know then where I might be able to find DAP Marine Resorcinol Glue or an euivalent in Canada?