The science of sealants

Sealant can be an emergency fix-it for leaks and loose fittings or a permanent bond; either way, you need to use the appropriate product for the task.

Unless you’re intimate with the intricacies of molecular bonds, navigating your way through the sea of sealants in a marine store can be confusing. Some are suitable for use below the waterline; others are restricted to deck jobs. Some do both.

Some are stronger or have better Uv-resistant features or are easier to apply. Some are geared to specific applications. Others cure quickly; others, slowly. Some can be painted over; others can’t be painted over. Many – but not all – are available in different colours.

To compound the confusion about a perfect bond, boats are built of many materials, as are their fittings. They include timber, melamine, glass, plexiglass, PVC, fibreglass, aluminum, zinc, bronze and stainless steel. In short, there’s no general-purpose sealant for all jobs. There are hundreds of sealants, and choosing the right one isn’t always straightforward.

It’s worth noting that many boat owners perceive and use sealants in a kind of crossover role as a sealant and an adhesive. Some sealants create an extremely tough bond and will often destroy the substrates to which they’ve been applied if you try to prise them apart. Use these with caution.

The basic rule of thumb is to ask for advice. Tell the chap behind the counter what you’re trying to achieve. Buying the right product almost always eliminates having to repeat the job in a few days, weeks or months.

WHICH TYPE?

Marine sealants fall into four basic chemical families:

Sika is the world’s largest sealant supplier to the marine

industry. Jon Grimley, Sika NZ’S target market manager – industry, says polyurethane products make up the market’s majority share, followed by a smaller segment of MS/SMPS, and then silicones.

“Polyurethanes are good broad-spectrum products. They can be used below and above the waterline,” Grimley says. “They’re versatile and generally work on all the common substrates found on boats – timber, aluminium, steel and fibreglass. Our most popular product, Sikaflex, is a polyurethane.”

Polyurethanes, he warns, are not good for plastic substrates such as Teflon, polyethylene or plasticised PVC, because these surfaces tend to have a low surface tension. Fortunately, plastic substrates are relatively uncommon on boats.

Most polyurethanes have exceptional adhesive characteristics but the strength varies considerably depending on the product. Polyurethanes such as 3M’s legendary 5200 are renowned for their mechanical strength and are excellent if you’re fixing something permanently. If there’s a chance you’ll need to remove a bonded fitting in the future, use a lower-strength polyurethane.

Polysulphide sealants are virtually unheard of in the marine industry these days. They were mainly used for caulking teak decks, thanks to their superior resistance to oil and diesel and aggressive deck cleaning agents. They’ve been superseded by a new range of polyurethanes with similar properties.

Silicone products are restricted to use on deck and the boat’s interior; ie, they cannot be used underwater. Their mechanical bond is much lower than that of a polyurethane sealant. The weaker bond is a bit of a moot point; the bond on deck fittings, for example, is usually provided by bolts/screws – the silicone merely provides a seal.

But silicones do tend to out-class the polyurethanes in one important area: they offer excellent UV resistance, which is why they are commonly used for sealing windows and hatches. They are resilient and have good resistance to fuels and chemicals. Unlike polyurethanes though, silicone sealants cannot be painted over and they can’t be sanded.

Improved Uv-resistance, says 3M, is what prompted the development of MS/SMP products. Based on a modified polyether technology, these behave more like a polyurethane than a silicone. Terry Bailey, managing director of Waipu’s Bailey Marine, is an agent for Simson, Bostik and Fixtech sealants. Most of these, he says, are SMPS and offer a few advantages over polyurethanes and silicones.

“Because they don’t contain any solvents or isocyanates there is much less of an odour and so they’re a bit easier to use. Similarly, without any evaporating isocyanates or solvents, they have a very low slump or settlement factor when they cure.”

Some SMPS, but not all, can be used below the waterline.

PREPARATION

One theme emphasised by all the suppliers consulted for this article is the importance of preparation: poor preparation is the major cause of a poor seal or adhesion. Failing to clean surfaces of dust, dirt, grease or oil makes adhesion difficult. A common issue is the remnants of release coatings on gelcoat – it seriously undermines a bond.

“Some surfaces,” says Sika’s Grimley, “need specific preparation. With something like stainless steel, for example, you normally need to scuff the surface with sandpaper before applying the sealant. Alternatively, you could apply a primer. This creates a film – a thin layer of resin over the surface – something compatible for the sealant.”

Many Diyers, he adds, unnecessarily complicate the application and cleaning of excess sealant. “A common practice, it seems, is letting the sealant partially cure before removing a bead of excess sealant with a knife. That can get messy.

“A better idea is to use masking tape, especially around windows and window frames. It’s very easy and much neater to pull off the tape before the sealant cures.”

This technique is particularly useful, he adds, when applying deck caulking. “Because polyurethane sealants can be sanded, cleaning a new deck of excess sealant is easy. But use masking tape on either side of the grooves when re-caulking an old, worn deck. That way you won’t have to sand the deck and remove any more timber.”

CURING

Another common pitfall for unwary users is applying a load to a newly-bonded fitting before the sealant is fully cured.

“All sealants are moisture cured,” says Grimley. “They take moisture from the air and become vulcanised – the product turns into a rubber. While many products are ninety per cent-cured in twenty-four hours, full curing can take up to seven days, depending on atmospheric conditions.”

Note that many manufacturers offer fast-cure variations on the standard products which is useful if you’re pressed for time. Curing time might also shape your thoughts when deciding on the most appropriate product for the job.

Consider deck caulking, for example, says Bailey. “You need a very specific product that flows easily and doesn’t cure too quickly to make sure it gets to the bottom of the seams.”

Users should also be aware, he adds, that some SMPS have a relatively short ‘open’ time. “They become tacky and start to skin quite quickly. If the sealant starts to skin before the two substrates are brought together, the bond will be compromised.

“This means you need to plan your job carefully, tackling small areas of a deck for example, rather than a large section all at once. It might be better to choose a sealant with a longer open time.”

Doing your homework before buying is an old cliché, but it’s particularly appropriate with sealants. The wrong product syndrome is best illustrated by the boat owner who cannot understand why he cannot eliminate a leak – despite pumping a litre of gunk into the hole. B

“Sealants... take moisture from the air and become vulcanised – full curing can take up to seven days…”