Guide to Varnishing

Boat Varnishing Made Simple...

Varnishing is something we often have to do, both on the boat and around the home. But why do we varnish wood? And how do we get a great finish? Our guide to basic varnishing projects tells you all you need to know…

Why do we varnish?


We varnish wood for various reasons, but for wood that will live outside our main objectives are to prevent water from entering the wood, and to prevent UV from the sun damaging wood (more about this in ‘types of varnish’. A third reason is that varnished wood is easier to clean - a smooth surface normally allows dirt to be wiped off more easily than a porous or rough surface. 


Types of Varnish


We can break marine or yacht varnish down broadly speaking into three categories: tung oil varnish, 1-part polyurethane varnish, and 2-part polyurethane varnish. 


Tung oil varnish is a good general purpose varnish, it is easy to use, dries in a reasonable time, and provides good protection from moisture and UV.


1-Part Polyurethane varnish is generally harder wearing than tung oil varnish. It still offers good protection from UV and moisture, but because it is harder it is often used on floors and areas prone to damage due to its added strength. It isn’t always the best product though, it can be so strong that it’s not ideal for really flexible items such as spars and can crack


2-part Polyurethane varnish requires 2 parts to be mixed together to create the final varnish prior to application. It offers great UV and moisture resistance, and is normally and extremely hard finish, so can be used in the areas prone to damage, or if we want the varnish to last for the longest possible amount of time. It isn’t always the best product though, it can be so strong that it’s not ideal for really flexible items such as spars and can crack as the wood flexes.


How to varnish like a pro.


Varnishing is sometimes viewed as a black art, but anyone should be able to achieve a reasonable finish by taking care in their work and not rushing the job. Our guide below really focuses on Tung Oil and 1-prt PU varnishes, but most of it applies to 2-part finishes too - there may just be some extra steps to factor in such as mixing the components. 


If the surface is already varnished, you must give the surface a gentle sand with 120 grit paper or rub down with a scotchbrite pad to make sure it is both primed and smooth ready for a new coat. We always try and use a similar varnish to what is already on the surface. If the surface is bare, again, give it a gentle sand, and make sure it is clean and dry. 


Plan your work - if you are painting a rowing boat for example, don’t paint the gunwails first as you will then need to lean over them to reach the bilges. Think about the steps you will take to get a full coat on to your object, and if necessary apply numbered bits of masking tape that you can easily remove showing you in what order to do various sections. Use a system to get complete coverage - varnish is normally clear, and you may lose track of your progress if you don’t do this. Make your route logical and repetitive to help you.


You can use most types of brush for varnish, and for most types of varnish either thinners no.1 or white spirit will act as a good cleaner/thinner. Foam brushes work particularly well with varnish, they have good absorption and leave very few brush marks.


Before your first coat clean the wood with thinners no.1 or white spirit to thoroughly degrease it - rub this along the grain with a cloth and allow to dry naturally. Most people thin their first coat on to bare wood, this allows it to soak in to the wood more, creating a more stable base on which to build up coats. This can often raise the gran slightly however, so another light sand after this first coat is recommended. 


Always allow your coats to dry fully before applying the next coat. Drying times vary between products, and are normally written on the tin. These drying times can change due to temperature - normally the warmer the temperature the faster the varnish dries, so taking your project indoors can speed things up. The more coats you can get on to a project the better the varnish will normally look, and the smoother the finish you will end up with. We normally recommend a minimum of two coats, but up to 6 or even more can be applied for a really amazing and durable finish.


Lightly sand or rub your workpiece down between coats with either sand paper (300-400 grit), or a scotchbrite pad. This may sound counterproductive, but it removes drip and brush marks, and gives your next coat something to grip on to that isn’t a perfectly smooth surface. 

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