Spar varnish

Overview

Spar varnish is varnish formulated for exterior or marine use, with UV absorbers or inhibitors added. The USDA Wood Handbook says spar varnish carries UV protection and that clear exterior coats last about 10 years at best. In practice the name covers very different products: Total Boat's Gleam is a one-part spar varnish and Lust a linseed-oil-modified polyurethane, while Varathane's Triple Thick and Minwax's Helmsman spar urethanes are water-based.

Pros

• UV protection is built in: Total Boat says Lust has heavy UV stabilizers (maker claim), and Varathane and Minwax list UV inhibitors.
• Gleam is brushable, rollable or sprayable, and Lust recoats in about 1 hour at 72 F with no sanding between coats (maker data).
• Water-based versions clean up with soap and water (Varathane) and are sold as mold- and mildew-resistant (maker claims).
• Varathane Triple Thick is described by the maker as 3X thicker than its other water-based spar urethane, needing one coat; this is a maker claim that we have not tested.
• Traditional alkyd/phenolic marine varnishes with UV absorbers also exist; see Alkyd varnish.

Cons

• Needs many coats: Gleam 4-6 minimum, Lust 6-8 recommended, Helmsman 3-4.
• Gleam must be sanded if recoated after 48 hours (maker).
• Ambers: Gleam is clear amber and Lust cures amber (maker data).
• The Wood Handbook's ceiling of about 10 years for clear exterior coats means all of them need to be recoated.
• The Gleam spec sheet says it is not food safe or food-contact safe once cured.
• Pettit's Captain's Varnish is not for use below the waterline; check each product's limits.

Suitability

Suited to exterior doors, railings, trim and boat brightwork, on species such as Western red cedar, White oak, Alaska yellow cedar and Meranti. The Varathane and Minwax water-based versions are sold for boats above the waterline, railings and patio furniture. Seal end grain and joints (Helmsman instructions). For broader guidance see Outdoor finishing.

Coats and application

Helmsman: do not thin; 3 to 4 coats, 4 for exterior or unfinished wood; seal end grain and joints; recoat after 2 hours. Gleam: overcoat after 1 hour at 80 F, 3 hours at 65 F and 8 hours at 50 F; sand if more than 48 hours pass. Lust: up to 5 coats a day, 6 to 8 recommended. Prepare with Sanding schedule and follow each can on thinning and brush choice.

Compatibility

• GF's rule for general finishes: oil-based must dry 72+ hours before a water-based topcoat, and water-based 24+ hours before oil-based; check the can before mixing oil and waterborne products (Water-based polyurethane).
• Over stain, let the stain dry and follow the can.
• Over a worn film, sanding and recoating is normal; Pettit's data gives 2 coats over existing varnish for its Captain's Varnish.

Care and repair

Exterior clear finishes need inspection and recoating (Wood Handbook: about 10 years at best).

Safety and environment

Solvent-based spar varnishes are flammable; ventilate and treat rags with care (Oily rag safety). Total Boat's Gleam VOC is under 475 g/L. Water-based Helmsman's label lists NMP and a reproductive-harm warning (Prop 65). The Gleam spec sheet says it is not food safe once cured; see Food-safe finishes.

Related

• Alkyd varnish: traditional varnish
• Oil-based polyurethane: interior equivalent
• Waterborne acrylic and urethane-acrylic topcoats: exterior waterborne topcoat
• Outdoor finishing, Oily rag safety