|Water-based and widely ignored|
|The need to adjust|
|What can be done?|
The need to adjust
HVLP doesn't feel all that different from high pressure. The necessary adjustments to achieve smooth results without orange peel or runs aren't very great, so finishers made the switch relatively painlessly. But the adjustments necessary to achieve good results using a water-based system instead of a solvent-based system are considerable:
***Water-based finishes run and sag on vertical surfaces far easier than finishes that thin with lacquer thinner.
Though formulators have managed to reduce some of the tendency, they will never duplicate the run resistance of lacquer because of the nature of lacquer thinner. It's made of six or more solvents that evaporate at different rates, causing the finish to seize up quickly on the surface while still retaining enough solvent for the finish to level out. With water-based finishes, the finisher has to adjust his or her spraying technique to compensate for its propensity to run.
***Water-based finishes raise the grain of the wood.
Again, formulators have managed to reduce this tendency somewhat, but it will never reach that of solvent-based finishes. Most finishers have learned to ignore raised grain by "burying" it under the sealer coat and then sanding the surface level; but more sanding is required so there is greater risk of sanding through.
***Water-based finishes are more sensitive to weather changes than solvent-based finishes.
There are widely available and well understood thinners that can be added to solvent finishes to compensate â€” primarily lacquer retarder and fast lacquer thinner. But there are no established, industry-wide, problem-solving thinners available for water-based finishes. Some suppliers provide solvents that help, but most don't. More on this later.
***Water-based finishes don't have the same intercoat adhesion ("burn-in") characteristics as most solvent-based finishes.
Formulators have improved the adherence of water-based finishes, but it's doubtful they will ever achieve that of solvent-based finishes because adherence depends so much on the ability of a solvent to penetrate and soften the surface of the existing coating. Finishers have to adjust their procedures to ensure good bonding and avoid "ghosting," or layering, when rubbing.
***Water-based finishes have a slower dry time than solvent-based systems (not counting varnishes) because of all the water that has to evaporate out of the finish.
Ovens can be used to speed up the drying (they can also be used with solvent-based finishes, of course), but ovens introduce problems of their own, including bubbles. Finishers have to adjust to the slower dry time.