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Marine Spar Varnish, What Is It and How Do I Apply It?

How to varnish your boat like a pro.

Marine Spar Varnish and Brightwork.  The varnished wood on many boats is referred to as “Bightwork”.  You can have just a few pieces of brightwork such as hand rails, toe rails, transom or your whole boat can be finished brite.  Doing the brightwork on a entire boat can be quite time consuming and may require extensive periodic maintenance but for the do-it-yourself persons can be a great source of pride.

There are three primary types of varnish, alkyd, polyurethane, and two part.  The alkyd varnishes are made from a base of natural oils such as linseed or tung oil.  These varnishes can also be a blend of the two oils.  With the addition of UV asorbents or reflectants the alkyd marine spar varnish can have a very high gloss, flexibility and good surface hardness, as well as water and weather resistance. Most marine spar varnishes are made from these bases.


Polyurethane varnishes are made from a polyurethane oil base.  Originally these varnishes had a more plastic finish which had very high flexibility and high abrasion resistance.  These varnishes did not contain UV inhibitors and were mainly used for interior finishes and flooring.

Two part varnishes refers to the newer types of clear finishes requiring a base and a hardener to be mixed.  Often these are Linear Polyurethanes and they have a extremely agressive solvent nature that prohibits their use over existing varnish or one part enamels. They have extremely high UV resistance and abrasion resistance due to their surface hardness.  However, in our experience they are somewhat lacking in flexibility which may lead to cracking when coated over a bare wood substrate. These are best used when a epoxy primer surface has been applied to wood first.  Lastly, in the event of damage to the wood, these two part systems can be very hard to make invisible repairs.


Here at Classic Craft we continuously test varnish in the outdoor enviroment to see how well they are holding up.  We compare our own tests with testing done by others, such as  the Practical Sailor Periodical, to verify that we are using and recommending the finest finishes available.

Application is pretty straight forward.  We do not recommend the over thinning of varnish as some persons do.  Wood is much like a mesh screen in that it will pass certain size molecules and not others.  Remember a wine barrel passes very little water over great periods of time and that the water molecule is one of the smallest and most universal solvents there is.  So why would anyone expect large molecule solvents and thinners to penetrate the wood any deeper.  Also, excess thinning can lead to solvent entrapment under the surface dried varnish and later bleed back into the varnish attacking it.  Follow the manufactures directions on thinning they really want you to have good results with their product and they will not steer you wrong.

We thin our first coat of varnish 10% and subsequent coats are never thinned.  Applying varnish this way will lead to quicker surface build and less porosity in the final varnish film.  If you are applying in cool weather we recommend heating your project to a minimum of 55 degrees farenheit and also placing the varnish in a pan of hot water to warm it.

In really hot climates the varnish may dry to fast not allowing for a proper flow out.  At this time you may want to add a retarded to increase drying times.  We like to use the Penetrol product for this purpose or a very small quantity of boiled linseed oil.