Marine Spar Varnish
A one part polyurethane, Marine Spar Varnish is a very durable coating for wood, when applied well. It has superior scuff and UV protection. It can be applied by brush, roller, or spray. Available in quarts or gallons in high gloss or satin finish. Coverage is 400-500 sq. ft/gallon (7.5-8.75 sq. m/l).
A properly applied and maintained polyurethane varnish will provide many years of life before the wood needs to be stripped and refinished. At least 10 years even in very harsh conditions, and likely longer if properly maintained in average conditions.
Surface preparation is key to a long lasting coating. This starts with sanding, followed by staining, if desired, then 1-2 primer coats of S-1 Sealer on dry wood (12-16% moisture content, depending on species). The wood should be sanded with whatever grit is necessary to start with, ending with #220, and the sanding dust removed without leaving any residue. Dry brush or vacuum is best.
If the wood is to be stained, oil or water based stains can be used. Apply to the bare wood, before coating with S-1. Be careful to follow the stain manufacturer's directions, and apply the first coat of S-1 Sealer whenever the first coat of varnish is recommended.
Follow the cured S-1 Sealer with 4-6 coats of Marine Spar Varnish. More coats can be applied if a deep finish is desired, but more coats increases the risk of surface cracking. Sanding between coats isn't necessary, if subsequent coats are applied between 12-24 hours. Sand with #400-#600 grit to remove imperfections before the final coat, or if previous coat was applied longer than 24 hours. Being careful to remove any sanding dust as noted above.
To get the best finish, a high quality brush is best. High quality varnish brushes are expensive, but give the best results, and if cleaned properly, last a long time. If rolling, a medium nap, solvent resistant roller works best. Three eighths-inch lambswool, or equivalent. If applying by sprayer, use a fine tip nozzle. Some thinning may be necessary to get the varnish to brush, roll, or spray well. Thin varnish up to 10% with mineral spirits, especially when spray applying. Whatever the application method, avoid applying in direct sunlight if possible, especially in hotter temperatures. Apply in temperatures between 50 and 95 deg. F. (10-35 deg. C.) 10-12 days is required for full cure before putting into hard service.
Lastly, proper application means leaving no bare spots, or thin areas, which can allow moisture to get in under the varnish. Be careful at hard to reach areas, and joints. Either wood to wood, or wood to another material.
While this will be a very durable finish, if exposed to significant UV, the surface will dull over time. Depending on the amount of UV, that could be as soon as 6 months, although that is a very severe case. Whenever this happens, a scuff sand and re-coat is required to maintain the base coats.
Maintenance for abrasion damage is similar. When scuffing removes the gloss, sand with #400–#600 grit, and apply a new top layer.
Occasionally, the varnish will crack, primarily from movement of the wood at a joint. It is important to catch these early with a periodic inspection. If caught before the moisture has a chance to stain the wood and cause the finish to lift, careful spot sanding and re-coating can fix the issue before it becomes necessary to strip and refinish the wood.
|Total Solids by Weight||50%|
|Color||Very Light Amber|
|Coverage||400 – 500 sq. ft/gal (7.5 – 8.75 sq. m/l)|
|Minimum Application Temperature||50°F (10°C)|
|Maximum Application Temperature||95°F (35°C)|
|Tack Free @ 77°F (25°C)||4 Hours|
|Re-Coat Time @ 77°F (25°C)||12 – 24 Hours|
|Dry-Through Time @ 77°F (25°C)||48 Hours|
|Full Cure for Hard Service||10 – 12 days|
|VOC Content||440–450 grams/liter (3.75 lb/gal)|
|Thinning||Thin varnish up to 10% with mineral spirits, especially when spray applying.|
|Clean up||Lacquer thinner or mineral spirits|
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