Different varnishes do different jobs. Understanding their properties will help you decide what's the best boat varnish for your woodwork. Rupert Holmes explains


Varnishing often appears to be a simple task, but there’s a bewildering choice of boat varnishes on the market.

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. Find out how to get the best finish with these 8 top tips for varnishing.

Basic types of varnish

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

Solvent-based varnish

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.

Hempel Dura Gloss Varnish is a urethane modified alkyd, ideal for interior use

Hempel’s Dura Gloss Varnish is a urethane modified alkyd, ideal for interior use

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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 varnish

Epifanes Clear Varnish includes tung oil and alkyd

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 varnish

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.

International Perfection Plus is a well-known two-pot polyurethane

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. If you need to save the varnish overnight, take a look at this top tip on preserving two-part varnish in the fridge. 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

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.

Dealing with surface damage? Don’t delay!

Whatever type of varnish is used, don’t delay on sealing any areas of damage. The key priority is to keep water out, so don’t wait until you have time and suitable weather for a full repair – touch in the damaged area immediately. This may not have a silky smooth finish initially, but don’t worry. Even if it’s a few weeks before you’re able to do a proper fix you know there won’t be any further degradation.

Top tip

A useful tip is to clean out one of the small match pots DIY stores sell to enable customers to test household colour schemes and fill it with varnish. They have a brush built into the lid, which never needs to be cleaned, thus making a quick touch up of damage a task that only takes a couple of minutes.

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This feature appeared in the February 21 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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