Alkyd varnish

Overview

Alkyd varnish is a clear film made from an alkyd resin, often with drying oils, dissolved in mineral-spirits-type solvent. The USDA Wood Handbook lists alkyd, acrylic and polyurethane as the clear film finishes and says films are the easiest to clean. Wikipedia's comparison table gives alkyd varnish a yellowish or orange tint that pops the grain, good protection, UV-resistant versions available, and says it can be stripped with paint removers. Pettit's Captain's Varnish is described by its data sheet as an alkyd/phenolic varnish with linseed and tung oils.

Pros

• Good protection and a warm, grain-enhancing look (Wikipedia).
• UV-resistant versions are available (Wikipedia); Pettit's Captain's Varnish contains UV absorbers per its data sheet.
• Brushable and sprayable (Pettit); can be thinned 5 to 10% with the maker's brushing thinner.
• Can be stripped with paint removers (Wikipedia).
• The Wood Handbook says clear exterior coats on wood last about 10 years at best, a useful ceiling for any clear varnish outdoors.

Cons

• Needs many coats: Pettit says at least 4 on bare wood.
• Slow: Pettit's data sheet gives 2-4 hours to set, 5-7 hours tack-free and 12-20 hours to dry hard at 70 F; Mohawk's Rockhard varnish is dust-free in 4 hours and recoats at 14.
• Ambers; Minwax's Performance Series Fast-Dry Varnish is described as amber.
• Solvent-based and flammable (Pettit flash point 105 F; SDS lists aliphatic naphtha 25-50%).
• Pettit says do not use below the waterline, and do not shake the can.

Suitability

Good for interior furniture and tabletops, and (with a UV-absorbing marine-type product) for exterior woodwork, on Red oak, White oak and Hickory, and for softwoods such as Western red cedar. For hard-wearing floors and tables, polyurethanes are the usual alternative (Oil-based polyurethane). For exterior parts see Spar varnish and Outdoor finishing.

Coats and application

Pettit's data sheet: brush or spray; do not shake; thin 5 to 10% with its brushing thinner; at least 4 coats on bare wood, 2 over existing varnish. Rockhard: scuff-sand with 320 before recoating; mineral spirits reducer. Mohawk's data: handle and recoat at 14 hours, use at 48 hours, rub at 72 to 96 hours (Rubbing out a finish). Minwax Fast-Dry: touch dry 30 minutes; recoat or scuff-sand within 3 to 4 hours. See Sanding schedule for grit progression.

Compatibility

• Sand lightly between coats; the sheets say to scuff-sand (Mohawk, Minwax).
• Over oil stains let the stain dry (Oil-based pigment stain); follow the can.
• Minwax's lacquer label says not to apply lacquer over polyurethane finishes; see Nitrocellulose lacquer.
• Water-based finishes over alkyd need adequate drying time: GF says 72+ hours for oil-based finishes (Water-based polyurethane).
• Seal knots with shellac (Wood Handbook): see Shellac, Dewaxed shellac.

Care and repair

Wipe with a damp cloth. Pettit's data gives a two-coat minimum over existing varnish, so a worn film can be recoated rather than stripped. Wikipedia says paint removers will strip it. Test on scrap first.

Safety and environment

Flammable solvents: keep ignition sources away and ventilate. Rags soaked in oil-based finishes can self-heat; follow the can and Oily rag safety. Pettit's VOC figure is 410 g/L, Mohawk Rockhard 408.61 g/L, against a federal limit for varnishes of 450 g/L (40 CFR 59). No food-contact claim is made here; see Food-safe finishes.

Related

• Spar varnish: UV-heavy exterior varnish
• Oil-based polyurethane: tougher, similar application
• Conversion varnish: catalyzed varnish despite the name
• Oily rag safety, Sanding schedule