Oil-based polyurethane

Overview

Oil-based polyurethane is a varnish-type film made with urethane-modified resin in petroleum solvent. Wikipedia's comparison table describes it as slightly ambering, with excellent protection against many substances, a tough finish, durable after about a 7 day cure, and easy when thinned and wiped on. It is the usual first choice for tabletops, trim and floors because it is widely sold, forgiving, and strong.

Pros

• Tough film: Wikipedia rates it excellent against many substances; GF rates its Arm-R-Seal "Highest" durability on its own test scale (maker claim).
• Warm, golden tone: Varathane describes a golden glow that deepens over time (maker claim); Wikipedia calls it a slight ambering.
• Forgiving to brush; Varathane describes a self-leveling formula.
• Can be stripped with paint removers (Wikipedia).
• Sold in satin, semi-gloss and gloss (Varathane).

Cons

• Slow: Minwax's fast-drying version needs 4-6 hours before sanding and 24 hours before light use; GF's Arm-R-Seal is touch dry in 12+ hours, recoat 24+, and cures in 30 days.
• Ambers; GF says not to apply its oil-based topcoat over light or white finishes. Pale maple and painted whites will warm.
• Sanding is required between coats (Wikipedia). Minwax says to stir and never shake.
• Strong solvent smell, flammable, and oily-rag fire risk.
• Interior use only for these products; for outdoor work see Spar varnish and Outdoor finishing.

Suitability

Suited to interior furniture, tabletops and floors (Minwax and Varathane list interior use) on Red oak, White oak, Hickory and Southern yellow pine. Ambering will show on pale species such as Soft maple and Hard maple, so a Water-based polyurethane is the usual alternative if you want to keep a pale look.

Coats and application

Prepare the surface (Sanding schedule). Stir, never shake. Minwax Fast-Drying: apply a thin coat with a natural-bristle or foam brush, let dry 4-6 hours, sand lightly with 220, apply the second coat, and allow 24 hours before light use; do not thin. On floors Minwax recommends three coats and 72 hours before heavy traffic. Arm-R-Seal: three coats by brush or roller (not sprayable), wipe off the excess, sand between coats with 320 or finer, and thin up to 10% mineral spirits if needed. Rub out after full cure (Rubbing out a finish).

Compatibility

• GF: let oil-based finishes dry 72+ hours before applying a water-based product over them.
• Do not apply lacquer over polyurethane (Minwax lacquer label).
• Wikipedia says dewaxed shellac is used where a topcoat must adhere (Dewaxed shellac, Shellac).
• Over oil stains (Oil-based pigment stain) let the stain dry before finishing; follow the can. The wiped-on version is covered in Wipe-on polyurethane.

Care and repair

Wikipedia says oil-based polyurethane can be stripped with paint removers. For wear, the usual route is to scuff the surface and add a coat; test on scrap first and follow the can.

Safety and environment

Minwax's label gives a DANGER warning that rags, steel wool and sanding residue soaked with the product may spontaneously catch fire, and tells you to place them in a sealed, water-filled metal container (Oily rag safety). Contents are combustible and contain aliphatic hydrocarbons; the label says the vapor is harmful and that the solvents can cause permanent brain and nervous system damage, so ventilate well. Federal VOC limits are 450 g/L for varnishes and 400 g/L for floor coatings (40 CFR 59); Arm-R-Seal's data lists VOC under 528 g/L for gloss. We read no food-contact claim on these pages; see Food-safe finishes.

Related

• Water-based polyurethane: clear, faster, lower odor
• Wipe-on polyurethane: same chemistry, thinned
• Alkyd varnish, Spar varnish: softer, outdoor-oriented cousins
• Oily rag safety, Sanding schedule