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#1 ·
Finishes and their Compatibility

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.

For example:

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
©2010-2015 Paul Jones - All Rights Reserved
 
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#3 ·
What are the differences between stains and dyes

What are the differences between stains and dyes?

Very simply put: With stains, the pigment tends to remain on the surface of the wood and lodge in the pores, while dyes penetrate deeply and color the wood from within.

Dyes

Dyes are colorants that are usually mixed in a carrier vehicle (solvents) such as mineral spirits, water or alcohol. The dyes used in woodworking are characterized as transparent, as they bring about color changes in wood without obscuring the figure. The molecular size of the dye particles is so small they allow light to pass through virtually unhindered. In simple terms, dyes are typically soluble salts. Once mixed with their proper solvent, dye crystals separate into individual molecules, which are vastly smaller than ground up pigment particles. Thus, dye can get into spaces where pigments cannot.

Water-based dyes are said to be less susceptible to fading (more lightfast) than alcohol-based dyes, while oil-based dyes fade the fastest. Alcohol-based dyes can be used to make toners and shaders by adding them to lacquer or shellac. By then gently spraying on very thin layers, you can blend unlike areas together or change the overall hue while retaining as much clarity as possible.

Stains

Stains are really nothing more than very thin oil or waterborne paints. That being said: dye stains are typically comprised of only dye and a carrier vehicle (solvents), stains are comprised of pigment, a carrier vehicle (solvents) and a binder. Using a thin varnish (oil-based) or acrylic latex (waterborne) as a binder, ground particles of natural and synthetic minerals are added to make stains. Stains should be stirred often to insure an even dispersion of pigment because the particles tend to settle on the bottom.

A special case in the stain family is the gel stain. As the name implies, gel stains have a much thicker consistency than liquid stains. The thickness of a gel stain depends on the brand you use. Some have the consistency of petroleum jelly, while others are more like peanut butter.

Because gel stains lie more on a wood surface instead of soaking into it, they more uniformly color porous and nonporous areas, unlike the light and dark blotches you sometimes get with liquid stains. Gel stains therefore produce a much more uniform color. Also, because they don't run or splatter, they're especially handy for applying to vertical surfaces.

The reason gel stains lie more on a wood surface instead of soaking into it is simple. There's less solvent in a gel stain. Because the solvent of a stain carries the color into the wood, the gel stain doesn't penetrate as deeply.

One other nice thing about gel stains is you don't have to keep stirring them as you work. Because they are thicker, the color pigment stays suspended instead of settling out in the bottom of the can. This means you get a consistent color from the top to the bottom of the can.

Differences between stains and dyes
©2010-2015 Paul Jones - All Rights Reserved
 
#6 ·
Wood Prep Before Finishing

Wood prep before finishing

The secret to perfect finish is proper sanding of your project. All surfaces should be clean and free from all dirt and oils. Prep sanding is done with progressively finer grits. On unfinished wood, prepare the surface by using medium grit paper first, and then progress to finer grades. With most raw woods, if you are hand sanding, start sanding in the direction of the grain using a #100-150 grit paper before staining and work up to #220 grit paper. You can make the sanding go faster with the use of Random Orbit Sander (ROS.

Soft woods such as pine and alder: start with #100 grit and finish with no finer than #220 grit
(for waterborne stains) and 180 grit for oil base stains.

Hardwoods such as maple and oak: start with #100 grit and finish-sand no finer than #180 grit
(for waterborne stains) and #150 grit for oil base stains. I have gone up to 220 grit on blotchier woods or those with high figure.

These are just some guidelines to start with, try various approaches on scrap samples to obtain the best possible outcome.

If you over sand you may seal the wood so much that it will not take a finish or stain well. Should you encounter end-grains (areas where the wood has been cut against the grain), such as the front and end edges of a table which tend to soak up more stain than other surfaces, sand with one or two grit steps up from the other surfaces. Make sure you address those end-grain areas with additional sanding to control this absorption of stain where if you do not they will be darker.

As mentioned above, you can use a ROS, along with and attached dust bag or vac to prep the wood surface and also between the coats of finish before putting on the top coat. The limiting factor is to use the correct grit of sandpaper when sanding between coats of finish.

I personally use the ROS between coat of finish with 320-400 grit to flatten any discrepancies in the applied finish levels followed by using a felt block wrapped with 400 grit sandpaper or 0000 steel wool depending on the shape, ie flat or complex. I always prefer to hand sand the final top coat starting with 400 grit working up as high as 600 grit. In both cases, always sand with the grain of the wood.

As a final step, I use a buffer and a buffing solution to blend the final coat just as they do in and auto body shop to increase the depth and reflection of light from the final finish. I also apply a coat of finishing wax like Staples Crystal Clear which to me adds a very nice mellow sheen.

The above process and result varies slightly depending on your choice of wood, finishing products rather they are waterborne or oil based, and application technique ie hand application or spraying.

Wood prep before finishing
©2010-2015 Paul Jones - All Rights Reserved
 
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