Using Polyurethane Finishes by Bill Nelson

Polyurethane is a liquid plastic resin that was developed in the 1950ís. Polyurethane is a very tough and durable finish that has many advantages as a wood turning finish. It is composed of vegetable oil acids, nitrogen compounds, and isocyanates in a mineral spirits carrier. The nitrogen compounds enhance flexibility, and isocyanates enhance the curing which makes the finish more durable. During the curing process a chemical reaction takes place with oxygen which causes the polymers in the compound to cross link and bond to one another very strongly. This cross-linking is what makes polyurethane so much more durable than other finishes and causes it to not be redisolved in mineral spirits. By comparison lacquer has a very weak bond because it is an evaporative finish with no chemical reactions taking place and can be easily redisolved in lacquer thinner. Because of the cross-linking there is no chemical bond between layers and the surface must be roughened up to achieve a mechanical bond between the layers of finish. One draw back is that polyurethane can cause a slight amber color on light colored woods. Due to environmental regulations water borne formulas are starting to appear. These formulations are not as durable as solvent based formulations but have some good points, quick drying, low smell, and water clean up. The reason they are not as durable is because the cross-linking occurs before the resins are put in the water carrier and when the water evaporates the molecules coalesce to make the film.

Solvent-based

* Polyurethane resins dissolved in mineral spirits

* Molecules cross-link for good wear and chemical resistance

* Amber tone; darkens with age

* Ventilation critical

* Temperature-sensitive curing

* Multiple (2-3) thin coats; sand and vacuum between coats

* Flammable solvents (volatile organic compounds)

*Recoat in 4-18 hrs.

Waterborne

* Polyurethane resin droplets suspended in water

* Droplets coalesce, provide good wear resistance but poor chemical bonds

* Clear/milky tone; non-yellowing

* Ventilation recommended

* Humidity-sensitive curing

* Multiple (3-4) thin coats; sand and vacuum between coats

* Noncombustible (soap and water cleanup)

*Recoat in 1-4 hrs.

How to achieve the high gloss polyurethane finish.

Sand the surface to at least 220 grit on lathe (possibly higher depending on the wood)

Apply one coat of polyurethane gloss (let dry 3-4 hours)

Sand the surface lightly with 220 grit stearate* paper

Apply addition coat of finish

Sand as before

After 4 coats you should be able to use 240 wet and dry paper with water ( do not cut through to bare wood)

When you have the surface level sand up to 600 grit with wet or dry with water**

Using a buffing wheel buff with white diamond compound until a high gloss is achieved

* Makers of polyurethane due to adhesion problems do not recommend stearate papers. Also do not use steel wool as there is a slight layer of oil on the wool to keep it from rusting and this will cause adhesion problems

** Do not cut through final finish layer, as you will see a line at the edge of the cut through area