How to Apply Marine Grade Varnish

by Kyle McBride

Marine varnish (spar varnish) protects wood from the effects of weather and wet environments. Wood that is finished bright (stained and varnished) is normally coated with several layers of varnish. The varnish dries clear and brings out the grain and natural beauty of the wood. The varnish layers form a hard shell that protects the wood from abrasion and impact damage. Suitable for use in exterior and marine applications, spar varnish is applied with simple techniques. Maintaining a good coat of varnish is a crucial part of wooden boat part maintenance.


Sand the area to be varnished with 120-grit sandpaper. Sand any rough areas out and leave a uniform finish on the work. This will open the grain to accept stain and give the work "tooth" for the sealer to bond to. Stain the wood at this point if desired. Allow the stain to dry fully.


Apply sanding sealer with a paintbrush. Coat the entire work surface. The sealer soaks into the wood and forms a bond with the subsequent layers of varnish. Allow the sealer to dry fully.


Rub the work down with the steel wool. The sanding sealer will raise the grain of the wood and lock it into place. Sand the work until it feels smooth to the touch and uniform. Remove all sanding dust by wiping the work with a rag.


Paint on a coat of varnish. Coat the work completely but not so heavily that it sags or runs. Allow the varnish to dry fully.


Rub the work down with a fresh piece of steel wool. Do not remove the sanding dust.


Continue applying coats and rubbing in between until the hollows created by the grain are all filled in, or until you get three coats of varnish. Do not rub the last layer of varnish after it dries.


Rinse all buckets and brushes well with lacquer thinner. Varnish that is allowed to dry in a brush will ruin it.


  • check Thin varnish can substitute for sanding sealer. Add small amounts of thinner to some varnish until it is thin enough to be soaked up by the wood. This will protect and bond with the wood the same as the sanding sealer.

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