One is brushing a product that contains the solvent for an underlying stain. For example, if you brush a water-based finish over a water-soluble dye that doesn’t contain a binder, you will smear the dye and cause the coloring to be uneven. The same is true if you brush lacquer over a lacquer stain. The lacquer-thinner solvent in the lacquer will dissolve the stain and your brush will smear it.
But there’s no problem spraying because no smearing can occur.
If you need to brush a water-based finish over a water-soluble stain or lacquer over a lacquer stain – to match a color, for example – you can apply a barrier coat of shellac or varnish in between. Shellac is usually the better choice.
Another exception is applying lacquer over varnish, though I don’t know why you would do this. The lacquer thinner in the lacquer may cause the varnish to blister. Spray light coats to begin with or apply a barrier coat of shellac.
Also, high-performance coatings such as conversion varnish, polyester and UV-cured finishes have special rules for application. Few readers of Popular Woodworking use these finishes, but if you do, be sure to follow the manufacturer’s instructions.
Coating an Old Surface
Almost any paint or finish can be applied over almost any old paint or finish as long as the surface is clean and dull.
It’s pretty obvious how bonding problems could occur if you apply paint or finish over a greasy or waxy surface, or over a surface that is covered with dirt (such as a deck). So the first rule is that the surface be clean.
Because there are two types of dirt, solvent-soluble and water-soluble, there are two types of cleaner: petroleum distillate (mineral spirits and naphtha) and water, or soap-and-water. Petroleum distillate won’t remove dirt on a deck, and water won’t remove grease or wax.
Some strong cleaners, such as household ammonia and TSP (available at paint stores), will usually remove both, however. Also, abrading the surface with sandpaper, steel wool or an abrasive pad will usually remove both types of dirt, along with the top surface of the coating – paint or finish.
The surface also has to be dull to get a good bond. Liquids don’t flow out and “wet” glossy surfaces well. Think of water beading on a car or glossy tabletop.
You can dull any surface using sandpaper, steel wool or abrasive pad, and many times you can dull the surface adequately with one of the strong cleaners – a little household ammonia in a bucket of water or TSP in water. Depending on the paint or finish you’re trying to dull, solvent-based “degreaser” and “liquid sandpaper” also often work. It won’t hurt to try; you can always follow with an abrasive.
Besides “wetting,” the reason a surface has to be dull is to create a “mechanical” bond between the new coating and the existing one. Dullness always indicates an uneven surface containing scratches, bumps or other irregularities that give the new coating something to lock into and grip. This is sometimes called “tooth.”
There are three situations, however, where coating over an existing coating can be problematic.
The first is when using any finish that contains lacquer thinner. This solvent can cause any old coating, even lacquer itself, to blister. To avoid blistering, spray several light coats and let them dry thoroughly before spraying fully wet coats. Or apply a coat of shellac first and then still spray a light coat of lacquer to begin with. Brushing lacquer is always risky because you can’t brush light coats.
The second is when coating over a high-performance finish that has been applied in a factory or professional shop. Bonding can be weaker even with a clean-and-dull surface.
Also, water-based finish and latex paint don’t bond as well to existing coatings as do solvent-based paint and finish.
Testing for a Good Bond
So you need to know how to test for a good bond. There are two ways.
The easiest is to press the edge of a coin into the newly applied coating after it has fully dried and drag the coin a few inches. You should just dent the surface, not separate the newly applied coating.
Another method is to use a razor blade to make perpendicular cuts into the coating about a millimeter apart and an inch long. Then press masking or other sticky tape over the cuts and pull it up quickly. The cleaner the cuts remain after removing the tape, the better the bond.
Of course, you should perform both these tests on an inconspicuous area or, better, scrap wood. PW
Bob is the author of “Understanding Wood Finishing” and a contributing editor to Popular Woodworking.