Finishes and their Compatibility
Almost any finishing product can be applied over any other as long as the “other finish” is dry and the product you’re brushing doesn’t dissolve and smudge the existing.
Let’s for arguments sake you are not using spray equipment and that you have made up and applied a water based PVA blotch controller as describe in my previous article Preventing Blotching Using A Wash Coat 1 to a cherry surface. You then apply a water-soluble dye for color and let it dry completely.
At this point, this is one of those “almost any finish” exceptions, as if you were to apply a waterborne finish or water-based paste wood filler<-(for what reason I do not Know), undoubtedly during the application you would smear the water-soluble dye. To insure this does not happen, you should apply a thin wash coat of Dewaxed Shellac as a barrier so the waterborne product will not have any effect on the dye.
After the waterborne finish or filler dried, You can brush on polyurethane alternated with waterborne, alcohol-based and mineral-spirits-based without any problems because each previous product was dry.
I’m sure you have come across cautions in woodworking books, instructions on product containers, and magazines instructing, you to “use a compatible product.” “What is compatible, stain, filler, glaze, finish, and what isn’t?”
And you’ve wondered, “So what is compatible with what?”
There are three entirely different situations which can be referred to by the term “compatible:”
• Mixing liquids with liquids
• Applying stains, fillers, glazes and finishes
• Coating over an existing finished or painted surface
First, let me say, the phrase, “use a compatible,” is a “cover-my-behind” dodge used by manufacturers and authors. Even if you use their procedures and then have problems, it must be your fault for using an “incompatible” product. The fault is on you to know what is compatible and what isn’t.
The truth is, in most cases, it’s obvious which liquids mix. Almost any finishing product can be applied over any other as long as the previous is dry. And almost any finish can be applied over almost any old surface as long as it is clean and dull.
What about Liquids
Most products we use in finishing are waterborne or mineral-spirits-based and the following is true: All waterborne mix successfully, and all mineral-spirits-based mix successfully. But the two cannot be mixed together.
The rule for coating successfully over an old surface is that the existing surface has to be clean and dull. So before applying another coat of finish to this 50-year-old lacquered cabinet door, wash it with household non-detergent ammonia and water. Ammonia cleans kitchen grease and dulls most finishes in one step.
It is easy to know when two products do not mix, they separate. Therefore it’s wise to use a glass jar for mixing if you have any question, so you can see what is happening.
What about Finishes
Most any finishing product rather they be a stain, filler, glaze, or finish can be applied successfully over any other finishing product, as long as that product is dry. The only exception is wax (including residue wax from paint strippers). This includes every finish over boiled linseed oil, and water-based finishes over oil stains. You should give the oil-based product a week to a month to dry in a warm environment, but once dry almost every finish will bond fine without problems.
So what about the uncommon exceptions to this rule?
One is applying a product with a brush 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.
At this point I will mention for those who spray; there’s no problem spraying because no smearing can occur.
As I mentioned above; 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 should apply a barrier coat of Dewaxed shellac.
Another exception is applying lacquer over varnish, for what reason, who knows. The lacquer thinner in the lacquer may cause the varnish to blister.
Again for those who use sprayers: Spray very light coats to begin with allowing the solvent to flash off or apply a barrier coat of Dewaxed shellac.
At this point I must mention high-performance coatings such as conversion varnish, polyester and UV-cured finishes which have special rules for their application. Be sure to follow the manufacturer’s instructions as they are beyond the scope of this piece of writing.
What about finishing 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.
A suggestion if you are refinishing: use the edge of a coin to test the bonding of a previous finish. If you can scratch off a layer rather than merely dent it, the finish isn’t bonded well and you must address this before you add a finish over one that is not bonded.
It may or may not be obvious how bonding problems could occur if you apply finish over a surface that is greasy,waxy or is covered with dirt. So the first rule is that the surface must be clean.
Some skeptics out there do not understand that there are two types of dirt, solvent-soluble and water-soluble. Correspondingly there are two types of cleaner: petroleum distillate (mineral spirits and naphtha) and water, or soap-and-water. Petroleum distillate won’t remove dirt and water won’t remove grease or wax.
Some strong cleaners, such as household non-detergent ammonia and TSP will usually remove both. Further, sanding 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.
Remember that the surface also has to be dull to get a good bond. Liquids don’t flow out and/or “wet” glossy surfaces well because there is nothing to have them grab onto. 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 a surface that gives the new coating something to lock into and grip. This is sometimes called “tooth.”
There are cases where coating over an existing coating can be challenging.
The first is when using any finish that contains lacquer thinner. This solvent can cause any old coating to blister, even lacquer itself. To avoid blistering, spray several light coats and let them dry thoroughly before spraying fully wet coats. Or apply a coat of Dewaxed shellac first and even then still spray a light coat of lacquer to begin with. Brushing lacquer is always dicey 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.
Finishes and their Compatibility
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-- Respectfully, Paul