Finish Compatibility | Popular Woodworking Magazine
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Find out which finishing products work well together.
I’m sure you’ve come across warnings in woodworking books and magazines telling you to “use a compatible product” – stain, filler, enamel, finish – and wondered, “What’s compatible and what’s not?”
The phrase “use a compatible” is a “back cover” dodge used by authors who have little knowledge of finishes. If you follow their procedures and then have problems, it must be your fault if you use an “incompatible” product. It is up to you to know what is compatible and what is not.
So what’s compatible with what?
Three completely different situations can be indicated with the word “compatible”:
• Mixing of liquids with liquids;
• Application of dyes, fillers, enamels and finishes; is
• Coating on an existing finished or painted surface.
As I explain each of these, you will see that the compatibility issue has been greatly exaggerated. In most cases, it is obvious which liquids are mixed. Almost any finishing product can be applied to any other as long as the previous one is dry. And almost any finish can be applied to almost any old surface as long as it’s clean and dull.
Mixing of liquids
Most of the products you use for finishing (or painting) are based on water or white spirit. All water based mixes successfully and all white spirit mixes successfully. But the two cannot be mixed together.
Wax mixed with paint only works well if you wipe off all the excess after each coat, just like with wax mixed with oil.
It’s easy to tell when two products don’t mix – they separate. For this reason it is advisable to use a glass jar to mix if you have any questions so you can see what is going on.
Application of finishes
Almost all finishing products – stain, putty, enamel, finish – can be successfully applied to any other finishing product, except wax (including residual wax from paint strippers), as long as the product is dry. This includes any finish on boiled linseed oil and water-based finishes on oil stains. You may need to let the oil-based product dry several days or a week in a warm room, but once dry any finish will adhere well without any problems.
Think about painting a piece of furniture that you finished several years ago with oil. You wouldn’t hesitate to use a water-based paint.
There are many quite rare exceptions to this rule.
One is to brush 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 does not contain a binder, you will smear the dye and make the color uneven. The same is true if you brush the lacquer on a lacquer stain. The thinner solvent in the paint will dissolve the stain and the brush will smear it.
But there is no problem with spraying because smudging cannot occur.
If you need to brush a water-based finish over a water-soluble stain or paint over a paint stain, for example to match a color, you can apply a barrier layer of shellac or paint in between. Shellac is usually the best choice.
Another exception is applying lacquer to paint, although I don’t know why you should do it. The lacquer thinner in the paint can cause the paint to bubble. Spray light coats to start or apply a barrier layer of shellac.
In addition, high-performance coatings such as conversion varnish, polyester, and UV-cured finishes have special rules for application. Few readers of Popular carpentry use these finishes, but if you do, be sure to follow the manufacturer’s instructions.
Coating of an old surface
Almost any paint or finish can be applied to almost any old paint or finish as long as the surface is clean and matte.
It’s pretty obvious how gluing problems could occur if you apply the paint or finish to an oily or waxy surface, or to a dirt-covered surface (like a deck). So the first rule is that the surface is clean.
Because there are two types of dirt, solvent-soluble and soluble in water, there are two types of detergents: petroleum distillate (white spirit and naphtha) and water or water and soap. Petroleum distillate will not remove dirt on a deck and water will not 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 needs to be matte to get a good bond. Liquids do not come out and “wet” shiny surfaces well. Think water beads on a car or a shiny table.
You can dull any surface using sandpaper, steel wool or abrasive pad and many times you can dull the surface properly 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 are trying to opacify, solvent-based “degreaser” and “liquid sandpaper” often work as well. It won’t hurt to try; you can always follow with an abrasive.
In addition to “wetting”, the reason a surface must be opaque 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 and grab onto. This is sometimes called a “tooth”.
There are three situations, however, where coating over an existing coating can be problematic.
The first is when using a finish that contains lacquer thinner. This solvent can cause any old coating to bubble up, even the paint itself. To avoid blistering, spray several light coats and allow them to dry completely before spraying fully wet hands. Or apply a coat of shellac first and then spray a light coat of paint again to begin with. Brushing paint is always risky because light coats cannot be brushed.
The second is when overcoating a high performance finish that has been applied in a factory or professional shop. Bonding can be weaker even with a clean, matte surface.
Additionally, the water-based finish and latex paint do not bond well to existing coatings such as solvent-based paint and finish.
Tested for a good bond
So you need to know how to test for a good bond. There are two ways.
The easiest way is to press the edge of a coin into the newly applied coating after it has completely dried and drag the coin a few centimeters. You should only dent the surface, do not separate the newly applied coating.
Another method is to use a razor blade to make perpendicular cuts in the coating about a millimeter apart and an inch long. Then press template or other masking tape over the cuts and pull it quickly. The cleaner the cuts remain after removing the tape, the better the adhesion.
Of course, you should perform both of these tests on an inconspicuous area or, better still, on scrap wood.
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