Catalyzed polyurethane (2K)

Overview

A 2K (two-component) polyurethane is cured by a chemical reaction after you add a hardener to the base. Wikipedia's comparison table describes two-part polyurethane as giving stronger protection than regular polyurethane, durable once cured (generally in under an hour), low or no VOC, generally sprayed, irreversible, easy to sand, and with sanding not needed between coats. It also says to clean equipment immediately.

Naming warning: General Finishes sells a water-based "Enduro Conversion Varnish" that it describes as a post-catalyzed, two-component urethane using an NCO catalyst. That is not the same chemistry as the acid-catalyzed conversion varnish defined by the EPA (alkyd with amino resins); see Conversion varnish.

Pros

• GF rates the Enduro Conversion Varnish at High durability (maker rating: Koenig 95, Taber 20.4 mg, water 5, chemical 101).
• Cures in 7 days on GF's sheet, against 21 for its single-component water-based lines.
• Low VOC: under 210 g/L on GF's data; the federal varnish limit is 450 g/L (40 CFR 59).
• Mohawk sells EnduraGreen for interior wood needing strong chemical and abrasion resistance, with sheens from 10 to 80 (maker).
• Finishworks says its AristoThane is isocyanate- and formaldehyde-free (maker claim).

Cons

• Mixing and pot life: GF says to mix up to 10% catalyst by volume (3.2 oz per quart), stir 2-3 minutes by drill or 5 by hand, mix only once, and leftover solidifies after 6 to 8 hours.
• Spray only on GF's data; needs spray equipment (Spraying with HVLP).
• Irreversible once cured (Wikipedia); repair is by sanding and recoating.
• Wikipedia: clean equipment immediately before it cures.

Suitability

For tables, desks, cabinets and other high-wear interior surfaces on Red oak, White oak, Hard maple and Hickory, where a standard polyurethane may not resist wear. GF lists the Enduro Conversion Varnish for interior and exterior use. The mixing, pot life and spraying steps make it a professional-shop product. For simpler options see Waterborne acrylic and urethane-acrylic topcoats and Water-based polyurethane.

Coats and application

GF's Enduro Conversion Varnish: spray three coats, recoating after 1+ hour (touch dry in 30+ minutes); light use after 72 hours and full cure at 7 days. Wear gloves and eye protection when mixing, and GF says self-contained breathing apparatus when spraying. GF's general notes apply: sand raw wood no finer than 220, thin only per the sheet, wet films of 3 to 5 mils with a 1.1 to 1.3 mm HVLP tip, and no steel wool. Mohawk's EnduraGreen catalyst is a separate product (maker says it is compatible with EnduraGreen products and helps the curing and hardening); follow its sheet for the mix ratio.

Compatibility

• Use only the catalyst the maker specifies for the product (GF's NCO Catalyst; Mohawk's EnduraGreen Waterborne Catalyst).
• GF: oil-based finishes need 72+ hours to dry before water-based over them; water-based 24+ hours before oil-based.
• GF: do not use water-based products with linseed or Danish oils; do not clear-coat over white or light paints.
• On sealed or pre-finished surfaces: scuff clean, wipe with 50:50 denatured alcohol and water, sand lightly and test adhesion (GF).

Care and repair

Cured film is irreversible (Wikipedia), so a repair means sanding through and respraying; sanding is easy, per Wikipedia. Test on scrap. Wipe with a damp cloth.

Safety and environment

GF's instruction is a self-contained breathing apparatus when spraying and gloves and eye protection when mixing. We did not find a more detailed health summary in the sources read; read the SDS before using any catalyst. Mohawk's EnduraGreen page carries a Prop 65 warning. Finishworks says AristoThane is isocyanate-free (maker claim). No food-contact claim is made here; see Food-safe finishes.

Related

• Conversion varnish: acid-catalyzed alternative with the same kind of use
• Pre-catalyzed and post-catalyzed lacquer: simpler catalyzed lacquer
• Waterborne acrylic and urethane-acrylic topcoats: one-part waterborne
• Polyester and UV-cured finishes: factory-cured systems
• Spraying with HVLP