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Water-based Finishes
  Water based finishes have been developed over the last 15 years or so, as a response to increasing sensitivity to health hazards and pollution. They are essentially a suspension. They include acrylic (which makes them clear hard and brittle) and urethane (which makes them tough, scratch resistant, and heat resistant) and use water as a solvent. Other chemicals are used to improve flow, keep the solids in suspension, slow curing and flatten the surface. 

 

About water-based finishes

    The majority of water-based finishes act as lacquers, as they cure by solvent evaporation. Like other lacquers, they are composed of resins and solvents, the main solvent being water. To get the resins compatible with water, other chemicals are used to create an emulsion, in which water is essentially the carrier. These are called surfactants. The main one is glycol ether, which is compatible both with water and the resins. The water evaporates after application, leaving the resin and solvent (glycol) to fuse or 'cure' on the surface. Once the glycol evaporates, the resin droplets form a smooth, hard finish.

What are the pros and cons of water-based finish?

Pros:

  • high in solid content for quick build up
  • fast drying
  • minimal solvent fumes
  • easy to clean up after use
  • very scuff resistant
  • non-yellowing
  • non fire hazard
  • less polluting
 Cons:
  • bland appearance
  • application difficulties in humid weather
  • raises the wood grain
  • no better than other finishes in terms of heat, acid, water/humidity resistence
  • hard to remove if allowed to dry
  • easy to get 'witness lines' when polishing
  • finish sensitive to low temperatures when stored

Applying Water-based Varnish

   Surface Preparation: In most cases you will need to raise the grain of the wood first, before you apply a water-based finish. Sand it to 180 grit, and use a sponge or cloth to wet it. After allowing it to dry overnight, sand with 220 grit.

  You do not need to raise the grain if.....

  • you use shellac as a sealer coat first
  • you use an oil-varnish stain
  • either will seal the wood and stop the w/b raising the grain.

   Using Water-based Finishes with Dyes: Water based finish tends to 'pull up' some dyes into the finish. The simplest solution if dyes are in use, is to seal with shellac before applying the finish. Only de-waxed shellac should be used for this sealer coat, to prevent any adhesion problems. The use of shellac as a sealer will also 'warm up' the wood in terms of colour tone, combating the 'washed out' look of water-based finishes.

   Colour: The colour in water-based finishes has a blue tint to it, which makes the wood look pale, cold and washed out. As mentioned, sealing with shellac helps prevent this. But if you otherwise want to apply colour, there are several options.

  •   You can apply a thin glaze of UTCs (universal tinting colour), or a water-based stain between coats. Choose a warm orangey-brown or burnt sienna. Dilute the colour with water and apply, wiping off the excess until the desired effect is reached. Let the dye dry overnight before applying more finish.
  •   Alternatively it is possible to add dye to the finish itself. This is a little tricky as you never know how much of the finish you will need to tint. It will 'warm up' the  look of the finish, but it will not accentuate the grain as well as dye applied directly to the wood.
  •   To add colour to a varnish, mix a little with water first. Never add the dry powder to the finish, as it will not dissolve properly. Remember, if you add a little and it is not enough, you can add more. If you add too much, you cant then remove any.

  Sanding between coats:

  •   There are no problems here, though some experts recommend NOT using searated paper or wire wool.
  •   Wait until the surface is hard enough to wet-sand. (a couple of hours in good weather, a bit longer if its humid -  you will know if you have gone too soon, as the paper will clog).
  •   Use 240 or 320 grit wet and dry with water as a lubricant
  •   Make sure after sanding, that all the residue is well cleaned. Use water or water-based glass cleaning fluid. The reason is that once dry, water-based finish will NOT re-dissolve into its solvent. If you you do not clean well, any bits will lodge in the finish.

   Brushing: The handling of water-based finishes depends on their formulation. Some foam excessively, others do njot flow out well and leave brush marks, whilst others take much longer to dry. Most of these problems can be overcome by applying thinner coats, or by changing the brush. The best brushes to use for this finish, are 100% synthetic filament (Chinex or Taklon for example) which is very fine and soft.

    First Coat: You can brush on water-based finish. Load the brush by dipping to half the depth of the bristles, start near an edge and stroke towards it. On reaching it, return to the start point and work in the opposite direction. Try to get the varnish to flow off the brush slowly. Use the tip of the brush to smooth out any irregularities as you go along. Do NOT go back over brushwork otherwise. Overlap the brush strokes slightly. If you leave it alone after finishing, it should level itself out.

   Top Coats: Because of the high solid content of water-based finishes, you may get a durable finish with just 3 coats.

  •   If you intend to 'rub out' to a shine, use several thinner top coats. You are then less likely to get 'witness lines' where you have rubbed through the top coat.
  •   Don't leave finish to cure for more than a day between coats, or you may have adhesion problems. If this is unavaoidable, then scuff the cured coat lightly before applying another.
  •   If you are using water-base to fill your pores, you may well need more than 3 or 4 coats.

   Rubbing Out:

  •   As with normal rubbing out. Level and wet-sand to 800 grit, but use water as a lubricant.
  •   Meguiars rubbing compounds work well to bring up a shine. Experts suggest rubbing compounds #1, #2 and then #10. I have had good success with Meguiars 'Deep Crystal' car polish.