Water-based polyurethane

Overview

Water-based polyurethane is a film finish in which urethane (often blended with acrylic) is dispersed in water rather than dissolved in petroleum solvent. Wikipedia's comparison table gives sheens from 1% to 80%, excellent protection, fewer VOCs, and notes that fast drying demands care to avoid bubbles. Brands differ: General Finishes' High Performance is a urethane/acrylic blend, Minwax sells Polycrylic and a water-based oil-modified polyurethane, and Varathane sells an Ultimate water-based polyurethane.

Pros

• Dries clear. Minwax says Polycrylic is milky in the can and dries crystal clear, and recommends it for light woods such as maple, ash and birch (maker claim). Wikipedia notes newer products can be UV stable if so labelled.
• Fast recoat: 2 hours for Minwax and Varathane; General Finishes lists 30+ minutes touch dry.
• Water or soap-and-water cleanup (Varathane, Minwax).
• Low odor (Varathane, Minwax Wipe-On water-based).
• Lower VOC: General Finishes lists High Performance at under 175 g/L (dead flat under 225) against a federal limit for varnishes of 450 g/L (40 CFR 59).

Cons

• Wikipedia says the thick finish can leave a plastic feel.
• Fast drying means lap marks and bubbles if you overwork it.
• Sanding is required between coats (Wikipedia; Minwax says 220 between coats).
• No steel wool: both Minwax and General Finishes warn against it on water-based finishes.
• Wikipedia says about 2 days to durable; General Finishes lists 21 days to full cure and 7 to 10 days before light use, so treat it gently for weeks.
• Minwax Polycrylic is not recommended for floors, and not over red mahogany stains.
• The sources disagree on cure time; follow the can for your product.

Suitability

Good for interior furniture, tabletops, cabinet doors and shelves on pale woods where oil-based amber would show: Hard maple, Soft maple, European ash, Yellow birch and Eastern white pine. Varathane and Minwax products here are interior products. For outdoor work see Spar varnish and Outdoor finishing. For floors, choose a product labelled for them (Minwax Polycrylic is not recommended for floors).

Coats and application

Prepare the surface (Sanding schedule). General Finishes: sand raw wood no finer than 220. Stir, do not shake (Minwax). Apply with a synthetic brush or pad in thin even coats; Minwax says 3 coats of Polycrylic with 2 hours between coats, and light 220 sanding between. Varathane: 3 coats by brush, 4 from the spray can. For spraying, GF gives wet films of 3 to 5 mils and a 1.1 to 1.3 mm HVLP tip (Spraying with HVLP); GF says thin with distilled water 5 to 10% if needed. GF says the finish is dry when it powders under 220 to 320 sanding. Over oil stain, Minwax says wait 24 hours before Polycrylic; with the Water-Based Oil-Modified Polyurethane, wait 8 hours after staining.

Compatibility

• GF: let an oil-based finish dry 72+ hours before applying water-based over it; let water-based dry 24+ hours before oil-based over it.
• GF: do not use water-based products with linseed or Danish oils.
• Do not use over white or light paints that may yellow (GF).
• On sealed or pre-finished surfaces: GF says to scuff clean with a non-woven pad and 50:50 denatured alcohol and water, dry 1 to 2 hours, sand lightly, and test adhesion.
• For sealers under it, follow the can; see Sanding sealer (and vinyl sealer).

Care and repair

Wait for the cure before heavy use (see times above). Clean with a damp cloth; GF advises avoiding commercial cleaners during the cure. Repair by scuffing the surface with a fine pad and adding a coat; test on scrap first.

Safety and environment

Water cleanup and low odor do not make it harmless. The Minwax Polycrylic label lists alkyl propanols, glycol ethers and N-methylpyrrolidone (NMP) and carries a California Prop 65 reproductive-harm warning (check the current label). Ventilate when sanding and spraying. No food-contact claim is made here; see Food-safe finishes.

Related

• Oil-based polyurethane: amber, solvent-based equivalent
• Wipe-on polyurethane: includes Minwax's water-based wipe-on
• Waterborne acrylic and urethane-acrylic topcoats: acrylic-based topcoats
• Catalyzed polyurethane (2K): a harder waterborne option
• Sanding schedule