Choose varnish
Which is the most appropriate varnish for the job in hand?
Varnishing often appears to be a simple task, but there’s a bewildering choice of products. An understanding of the qualities of different types of varnish is needed to choose the optimum one for each purpose. This is particularly true for exterior woodwork which has to withstand the ravages of sun, rain and frost.
There are only a few basic types of varnish, although in many cases hybrid products are available, particularly alkyd/ tung oil and alkyd/modified urethanes that can offer a wider range of desirable properties.
Traditional solvent-based one-pot alkyd varnishes are ideal for use with traditionally built wooden boats. The coating is soft but flexible, which makes it resistant to cracking when used on timber that expands when wet and contracts in dry summer weather. Additionally, the initial coats tend to soak into the timber effectively.
On the downside, alkyd varnishes lack resistance to abrasion and usually only one coat can be applied per day, which can make varnishing a drawn out process. The more expensive types are formulated to speed up cure times, improve gloss retention and generally last longer.
Tung oil is also frequently used in traditional varnishes: it soaks in well and, although a pure tung oil product will usually cure very slowly, the final finish is very tough. Polyurethane varnishes are a harder option with good wear resistance, but they lack the flexibility of alkyds. In addition, they tend to sit more on the surface of the timber, so often do not adhere as well to the substrate, which can make them prone to flaking and chipping. The lack of flexibility makes a pure polyurethane unsuitable for traditionally planked boats, but it can be ideal for plywood, as well as for cold moulded and strip planked hulls.
Polyurethanes are available as both one-pot and two-pot products, the latter giving a harder coating and faster cure, which allows multiple coats to be applied in one day. However, they are a much more expensive option.
Note that one-pot polyurethane can be applied over two-pot varnish but not the other way round.
Ultra-flexible single-pot polyurethanes, such as Coelan have an elasticity of more than 300%. It can also seal cracks, is highly resistant to impact damage and doesn’t need overcoating for several years and can last twice as long as traditional varnishes.
The biggest downside is that it’s four or five times more expensive than typical basic alkyd varnishes, so up-front costs are higher, although its long term performance means the total cost difference will be smaller.
Epoxy resin can also be used to coat timber, but generally has poor resistance to ultra-violet light, so a varnish needs to be applied over the top.
Beware inexpensive ‘yacht’ varnish sold in DIY stores – the low prices are usually easy to explain in terms of extended drying times or poor long-term performance when used outside.