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Preparing Wood for Varnishing - Interlux


By Pete Mathews, Interlux Sales Representative

There are nearly as many ideas about how to prepare wood and apply varnish, as there are types of wood. In this article I will share the knowledge I have gained from my own experiences and some tricks of the trade from professionals. For those interested in exploring the subject in greater detail, there are several very good books available on this subject at your local marine store.



If you're refinishing an existing piece of wood, the first step is to thoroughly remove all the old finish. This can be accomplished by chemically removing the old finish with Interlux Interstrip 299E semi paste. You can use brushes to apply the stripper and scrapers to remove the old finish, taking care not to gouge the wood during scraping. Finish the chemical stripping process by scrubbing with the Interstrip and bronze wool.

Another method is to sand off the old finish. If you're going to sand, be sure not to damage the wood in the process by improper use of sanders. Orbital sanders, and "DA's", (dual action or random orbital sanders, air or electric) are the most commonly used. An airfile can also be used on flat surfaces using caution to always work with the grain. Do not use grinders or belt sanders. These tools can do a great deal of damage in a very short time, causing more time to be invested in repairing the damage.

Once all the old finish has been sanded off, I recommend scrubbing the surface with bronze wool and Interstrip to be sure all the old finish is removed from the grain. If you are using a Methylene chloride based stripper, (we hope not) be sure to neutralize the stripper with denatured alcohol before continuing. I finish the process with a final cleaning with TSP (Trisodium phosphate) diluted in warm water, followed by a thorough rinse with plain water. The wood is then allowed to dry thoroughly before proceeding to the next step.

New wood should be sanded with 60-grit sandpaper until the wood is smooth and shows a consistent color. At this stage, either hand or machine sanding can be used.

Now the wood, either new or stripped, should be completely sanded with 60-grit sandpaper, then with 80-grit sandpaper, then finally 120-grit sandpaper, all hand sanded. Always sand with the grain at this point. If any circular motion is used, the stain, if the piece is to be stained, will show the sanding pattern. If not, even the varnish can show swirl marks from orbital sanders. After sanding wipe down the wood with Interlux 333 Brushing Liquid to remove all the sanding residue left behind.




If you are going to stain the wood, this is the time to do it. Interlux currently manufactures three Paste Wood Filler Stains; #42 Brown Mahogany, # 573 Chris Craft Red Mahogany, and #1579 Red Mahogany. These are Paste Wood fillers that must be diluted before using. The stain should be mixed with Interlux 333 Brushing Liquid until it reaches a soupy consistency. This is then applied with either a brush or rag (I prefer a rag). Apply the stain liberally in a sloppy fashion, trying to get it on as evenly as possible. It should then be immediately wiped off using a circular motion with a course material wetted with some of the stain. Old towels or burlap work well. This should be followed with a wipe down with a clean towel or burlap. Continue wiping until all the excess stain has been removed, but do not polish the wood. You should achieve an even color over the entire area.

Do not attempt to go back and re-stain an area. This will leave an uneven color that cannot be blended in. Once you are satisfied with the stained wood, allow the surface to dry for at least eight hours. I suggest trying the stain on a piece of scrap wood, of the same species, before tackling the whole boat.

The next step will be to seal the wood surface or stained wood surface. Here again, there are several ways of doing this. Perhaps the oldest and simplest method is to thin the varnish 30-50% with Interlux 333 Brushing Liquid. Use the varnish you intend to use as a topcoat. Some people will apply several coats, reducing each coat less to build up a base for the full strength coats of varnish to follow. Light sanding with 220-320 grit should be done between each of these coats followed by cleaning with a solvent wipe (333 brushing liquid). This system has the advantage of using a varnish with UV protection all the way into the wood. Be careful when sanding the edges and corners between coats. A 3M® Scotch-brite® pad works well for this.

Another method is to apply at least one coat of Interlux 1026 Interprime Wood sealer. Some people will apply two or three coats. Be sure to sand lightly with 220 to 320 grit sandpaper and clean the surface between each coat. Wet sanding is not recommended at this point, as it will raise the grain of the wood. This will make the task of getting a smooth surface more difficult. Again, this sanding should be done with the grain. The Wood Sealer is thinner than unthinned varnish and so will penetrate the wood further. It is easier to sand than the thinned varnish, but doesn't have the UV protection.


Varnishing over epoxy coatings is very popular today with more and more cold molded, laminated wood and "strip built" boats being built. Epoxies such as Interlux Epiglass HT9000 can provide an excellent base for varnish on boats built in this manner. The wood must be prepared the same way described above prior to applying the epoxy, with one important addition. After the epoxy has cured, you must remove all the amine blush from the surface of the epoxy before sanding. Amine blush is the waxy substance that forms on the surface of the epoxy as it cures. This is best accomplished with warm water. Some people prefer to add a little household detergent to this warm water. I always wait overnight after such a washing to see if the blushing has stopped. If it has not, repeat the process as necessary. It is vital to remove all the blush before sanding preparatory to varnishing. This blush can interfere with the drying, performance and longevity of the varnish if it isn't removed.

Preparing to re-varnish an existing finish is much easier. If the finish is in good condition with no cracks, chips or deep gouges in it, all that needs to be done is sanding and cleaning. If there is damage to the surface leaving black marks at joints or along edges, then perhaps going back to the beginning and removing the finish is in order. If not, the surface should be dry sanded with 220 to 320 grit sandpaper, 320 to 400 grit if wet sanding. Some people will then wash with a soft rag and clean water. If this is done the surface must be allowed to dry thoroughly. Then, wipe with 333 solvent and allow the surface to dry once more. The final step in any preparation process is to lightly wipe the surface with a tack rag to remove the last of the dust (hopefully).

There are other methods varnishers use to seal the wood prior to varnishing bare wood, such as wet sanding with oils or saturating with linseed oil. These systems are more advanced and outside the scope of this article. They can be found in the more comprehensive books mentioned before. But the methods described above are the standard methods used by many professionals and most amateurs. Any of them, if done properly and with sufficient attention to detail, will provide an excellent base for the varnish to come, and a varnish job to be admired when completed.

To learn more about interlux paint visit http://www.yachtpaint.com

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