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How to Paint - Primer Part 3 - Specialty Coatings

Updated on January 5, 2016

Specialty Coatings

Specialty coatings can be categorized by the function that they perform and the special materials they are used on. Wood, still being the primary building material, can also be covered and protected by a variety of special coatings. Other more exotic materials and needs are satisfied by a variety of specialized and unique products.

Let us first explore the various specialty coatings designed to be used on and to enhance the specific characteristics of wood.

Transparent Coatings

Varnishes

Defined as - a liquid that dries to a hard transparent coating - the most popular types of varnish on the market today are Alkyd or Urethane. However because all varnishes contain solvents they are slowly being replaced by such products as "Clear Acrylics." Varnishes dry hard to varying degrees of shine from flat too high-gloss. They are most often used on wood surfaces although some are formulated for concrete and others for metal.

Alkyd: varnishes are classified according to the oil content in their resins called "oil length." Varnishes are called "short" if they contain less than 15 gallons of oil per 100 pounds of resin. "Medium-oil" varnishes usually contain 18-25 gallons of oil per 100 pounds of resin. "Long-oil" varnishes contain more than 25 gallons. The long are softer, more flexible and take longer to dry. Used mainly for exterior work because they are more elastic and offer better weather resistance. Except for alkaline substrates, these varnishes will adhere to most exterior surfaces.

Short oil varnishes are harder, more brittle and faster drying. They are highly resistant to alcohols and acids, but because of their brittleness, are not recommended for wood floors. The ideal usage is for someone wanting a "rubbed" finish look like that on furniture.

Medium oil varnishes, on the other hand, are less brittle and ideal for wooden floors, interior wood trim and furniture .

Urethane: This is the toughest of all varnishes and is resistant to abrasion, solvents, heat, household cleaners, water, alcohol and chemicals. Most are flexible enough so they won't peel, chip or flake and have exceptional durability . Urethane's are perfect for high-traffic or heavy-use areas.

General purpose urethanes perform well on furniture, doors, trim and cabinets. Most adhere well to wood, ceramic tile and many other surfaces. When diluted some urethanes make good sealers however, always read the manufacturer's label before giving advice about diluting any coating product.

Clear Acrylics:

These are available in both solvent-based and water-based forms. The solvent-based varieties are favored by professionals. They are more durable than the water-based varieties. Still, with the rapid technological improvements, low solvent content and reduced VOCs, the water-based acrylics have increased in popularity in recent years.

Dry acrylic finishes are water-clear and do not yellow when applied to wood, unlike the amber-colored alkyd varnishes . Compared to alkyd and urethane varnishes, the clear water-based acrylics are generally less abrasion resistant. These are typically used in areas where abrasion resistance is not a requirement, such as furniture finishing. There are, however, a few newer formulations that are durable enough for use on floors.

Lacquers:

Lacquers are "cellulose" bathed in nitric acid to produce a resin called "nitrocellulose" (also called "cellulose nitrate"). These produce a variety of coverage features. Other resins used in manufacturing lacquer are cellulose acetates, cellulose acetate-butyrate, ethyl cellulose and, more recently, acrylics. Acrylics, both alone and with nitrocellulose are used in auto finishing because of exceptional gloss retention.

While nitrocellulose is the most important lacquer ingredient, it contains high levels of VOCs and recent legislation has forced a decline in usage .

Variations created with as many as 20 ingredients (different solvents, resins, pigments and plasticizers) in one product, account for the broad range of features available: Hard or soft, almost instant drying or slow drying, clear or colored, high gloss or flat sheen, thick or thin.

Most lacquers dry rapidly to a very hard film, are highly resistant to stains and chemicals but emit a strong odor and are extremely flammable. They also are susceptible to UV radiation. Urethanes offer better stain resistance but lacquers are easier to rework if the film gets marred.

Shellac:

While this product was covered in our section on primers it is also considered a specialty coating, Shellac film cures to a hard, durable finish with moderate abrasion resistance. Applied to bare, bleached or stained furniture, woodwork and floors, shellac provides a mirror-smooth finish. It is extremely sensitive to alcohol and incompatible with most urethanes. The widespread belief that shellac and water are incompatible is largely false; however, a shellac film can be damaged if water is forced into it. When used on furniture or floors a topcoat of wax will usually provide protection from moisture. In most applications, shellac offers adequate moisture resistance.

Finishing Oils:

Finishing oils may or may not be considered "coatings." Danish Oil, Linseed Oil, Tung Oil and Swedish Oil are known as the colored oil finishes and provide color and protection in one step. Many customers select them as low-luster finishes for fine furniture. They are not as resistant to alcohol and water as varnishes and acrylic. They are not recommended for applications on abuse-prone furniture such as coffee tables.

Tung Oil is the favorite of furniture refinishers. Linseed Oil is often substituted for Tung Oil because it is cheaper. Both give a mellow luster to wood. Linseed Oil, when applied to raw wood, will darken it and is not highly resistant but will be less likely to show scratches than varnish. Preparing and applying boiled Linseed Oil is a multiple-step process and one should consult a supplier before using

Stains

Officially defined as a dye that penetrates and colors a substance, stains are usually applied to either interior or exterior wood surfaces. The recent desire for more natural-looking woodwork has resulted in a greater demand for stains. The variety of them available to consumers today is vast and growing. In simple terms, stains are thin paints, specially formulated for application on bare wood. Generally stains do not protect wood as well as paint but they do afford certain aesthetic advantages by allowing the natural color, grain and texture of wood to show through. By way of contrast, paints and primers serve to provide smooth, uniform surfaces and are applied in heavier coats. Stains can also make one type of wood look like another, creating uniform color in a piece made of more than one kind of wood and in patched areas. Interior stains are most commonly used on furniture and woodwork. Exterior stains are primarily used for wood siding and shingles. Stains are often recommended for woods such as cedar, redwood and cypress, all naturally resistant to rotting.

Solid (Opaque):

Stains generally contain relatively low levels of pigment. Solid or Opaque stains are made with much higher concentrations of pigment. They tend to obscure both the color and grain of the wood, but not it's texture as paint does. Higher levels of pigment means that these kinds of stain come in a wider range of colors, including deep tones and pastels. The high pigmentation also protects against ultraviolet degradation and offer more durability . Normally applied in one coat it is important in some cases that a stain-blocking primer is used as an undercoat, especially for tannin rich woods like redwood and cedar.

Semi-transparent & Clear:

With semi-transparent stains their low level of pigment allows them to showcase the natural woods grain and texture . However, the reduced pigment levels provide less protection against the harmful effects of ultraviolet degradation. For the most part these stains are provided in natural or wood-tone colors . The semi-transparent stains are penetrating products that, in time, erode with weather. Because of the demanding nature of applying semi-transparent stains (no primer, one coat on bare wood) it is especially important to recommend a top quality product.

Pigment & dye:

The color in stains comes from either pigments or dyes. Pigmented stains rarely have the clarity of dye stains, they hide some of the stain-like quality and are seldom used on high-quality furniture. Carbon black and the iron oxide pigments provide transparent earth tones such as raw and burnt sienna, burnt umber and Vandyke

  1. Since the stain does not penetrate the surface, if the film gets chipped, the substrate will be unprotected.
  2. Varnish stains tend to run, sag and show lap marks if it is concentrated in heavy layers.
  3. The final film coat is not as durable as ordinary stain covered with a separate application of varnish .
  4. They also, do not provide the same grain clarity available from a stain to which varnish is applied as a topcoat.

Special Function Coatings

BITUMINOUS COATINGS

Used most often for waterproofing, roofing and to protect submerged metalwork, these coatings are also called Asphalt Roof Coatings and are made with either coal tar or asphalt. These types are thicker than ordinary paints and extremely resistant to moisture and oxygen. They may be reinforced by epoxy or urethane resins for improved adhesion, flexibility and better resistance to chemicals. Pure asphalt is not a paint, although it is a protective coating, usually applied to driveways, roofs, roads, etc.

ALUMINUM PAINTS

These paints contain particles of aluminum suspended in an oil or alkyd resin. There are two distinctly different types of aluminum paint ; "flake" and "non-flaking." Both types provide a shiny, metallic finish with outstanding durability.

Flake gives a glittery look like the metallic paint used on automobiles. Other flake formulations are designed specially for stoves, furnaces, etc. This type of aluminum paint contains more aluminum than the non-flaking kind and costs more.

Non-flaking this type of paint is typically used on exterior structures such as metal buildings, roofs, flashing, down spout, gutters, or on objects to give them an aluminum look. Aluminum paint adheres well to most galvanized steel and iron surfaces and is also used on asphalt siding, brick, stucco and concrete blocks. As a primer it solves stain problems that few other coatings can fix. For example aluminum paint and shellac are the only coatings that reliably adhere to creosote­ treated wood and seal the creosote stains in the wood. Aluminum paint can also fix resins in redwood, red cedar and mahogany, keeping them from bleeding through. Surprisingly though aluminum paints are seldom used as primers. One major drawback is that the colors change dramatically as the paint ages. Warning: the newer water-based aluminum paints have limited shelf stability and must be used prior to the expiration date.

WATERPROOFING COATINGS

These coatings applied to the exterior of basement walls will prevent water seepage. Usually solvent based, clear and containing 5% silicone they may be used on damp interior walls, cracked plaster and warped woodwork . A single application will reduce water seepage to a minimum and may last five to ten years . The silicone coatings are usually applied by spraying and must be applied liberally to ensure complete coverage and absorption. These coatings adhere to both unpainted and previously painted masonry substrates. Silicone coatings will not adhere to existing oil-based or alkyd­ based paints and can prevent paint from adhering.

FIRE-RETARDANT PAINTS

These paints slow down a fire by puffing up and forming a spongy insulating layer when exposed to high temperatures. This process is called "intumescence". While the insulating layer will char it will not flare or allow flames to spread along its surface, temporarily protecting any combustible material under the paint from burning.

The ingredients in these paints which help to protect against fire are: antimony oxide, zinc borate and phosphates - look for these substances on the labels. These products are used to paint furnace rooms, workshops, storage rooms, halls, stairways, closets, children's rooms and kitchens. They can keep wood studs and joists from reaching the kindling point and can be applied to bare wood, metal, wallboard, acoustical tile and existing paint.

Important: it is crucial to pay special attention to the label instructions and strictly follow directions to insure that the paint performs as intended.

WOOD SEALERS

Most of these are thin, solvent based liquids which serve one or more of the following purposes:

  1. To penetrate and close off pores to prevent the wood from absorbing subsequent coatings.
  2. To lock stains, chemicals and moisture in the wood to keep them from seeping out and discoloring or loosening subsequent coatings .
  3. To protect wood against external moisture damage.
  4. To fill in surface irregularities and create a uniform, even surface.
  5. To equalize the density of hard and soft grains in a single piece of wood . To allow a more even finish when varnish or paint is applied.
  6. To harden small fibers on rough surfaces, making sanding easier.
  7. To help with adhesion.

WOOD PRESERVATIVES

With the modern pressure-treated lumber available today lasting 30+ years, the demand for wood preservatives is declining. The Pressure-treated lumber contains wood green a chemical which resists rot, insects and disease. Still, untreated lumber may be used for exterior construction and in out-door furniture. Plus there will also be customers who wish to add preservatives regardless of lumber treatments.

Most wood preservatives are solvent-based, but some of the water-based products are just as effective. They also offer reduced environmental hazards and easier clean-up. When coating exterior wood above ground (decks, patio furniture, porches, etc.) always remember to apply preservatives first, unless the selected stains/paints already contain wood preservatives.

Special Surfaces

MASONRY

This category includes stucco, brick, concrete, plaster , cement, cinder block, stone, mortar and asbestos shingles. The two important characteristics to remember and addressed are: Porosity and alkalinity.

Porosity:

"Porous" building materials are substances that have gaps or pores in them which allows moisture and other environmental substances inside. If moisture is trapped

inside a porous material, eventually it will rise to the surface to escape and evaporate into the atmosphere. If the paint film applied to the surface of masonry material is permeable, it will allow the moisture to pass right through. A "permeable" film is one which permits the passage of liquids and vapors without damage to the film itself. If the paint is non-permeable the moisture will push the film away from the substrate and cause failure of adhesion. Because of latex's permeable nature moisture can pass freely and they adhere better to such material as masonry.

Alkalinity:

Alkaline substances contain alkali's or hydroxides of the alkaline metals. Saponification, or the turning of oil into a soap, may result if oil-based and alkyd­ based paints are directly applied to a masonry surface. This soap film will keep the coatings from adhering to the substrate.

There are two types of paint that are commonly used on masonry; powdered cement paint and latex. The powdered cement paints contain Portland cement, pigments and usually a small amount of water repellent. These paints are inexpensive and usually used in basements. They will add a thin layer of fresh cement to the walls and may contain fillers to help smooth the rough masonry surface.

Acrylic resin makes the best masonry latex paint, however, ordinary latex house paints are suitable for most masonry surfaces - assuming the surface has been properly prepared. Some of the newer concrete building materials (such as new light-weight concrete blocks) require special primers. Never the less, latex masonry paints hold up better and are easier to use than cement paints.

Elastometric

New coating technology has produced this specialty product for masonry that is becoming increasingly popular in the United States, while having been used in Europe for years. In simple terms, Elastometric coatings are paints that are applied in very thick films and which are specially formulated to stretch and bridge cracks. This type of coating will flex with the "movement" of a structure under extreme temperature variations and prevents moisture from penetrating thin cracks in the substrate.

To achieve the special balance of properties required, manufacturers use acrylic binders designed specifically for these coatings. They have a high degree of tensile strength and resistance to dirt. Top quality elastometric coatings offer good resistance

to moisture but also breathe , allowing water within the substrate to escape into the outside air. The coatings dry to a satin finish and provide, not only great protection, but a crisp appearance for years. It is very important to prepare the substrate properly, making sure that the masonry is clean and sound.

METAL PAINTS

There are many different coatings, varying m sheen and color, designed . for the specific demands of metals such as iron, steel, galvanized steel, aluminum, copper, brass, bronze, tin and terneplate .

Aluminum & Steel

Both of these metals will oxidize and corrode unless protected. When steel oxidizes rust and rust stains result. When aluminum oxidizes, a white powder develops on the surface and the metal itself develops pits. The only way to prevent such corrosion in either metal is to seal out the oxygen and water .

Corrosion-resistant paints

These coatings use one of three methods to prevent water and/or oxygen from reaching the substrate:

- Create a barrier - barrier paints contain pigments such as micaceous iron oxide, which reduces the coatings permeability . Air and water cannot pass through the film and reach the metal underneath.

- Galvanic process - Galvanic/Sacrificial paints are "zinc-rich" primers and contain as much as 80-92% zinc dust with a very small amount of vehicle . The galvanization process leaves a zinc residue on the surface for protection.

- Inhibitive method - Inhibiting paints contain such pigments as barium metaborate, zinc phosphate or strontium chromate. These pigments plug the small pores in the dry paint film and absorb any moisture that penetrates the coating surface, keeping it from reaching the substrate and causing corrosion .

Solvent based vs. water-based: Most of the specialized metal primers available today are oil or alkyd-based, while the number of latex paints designed for metals is growing. Also new self-priming or direct-to-metal coatings have been developed by the United States military for use on their equipment.

Copper, bronze & brass

These metals are known for their natural esthetic qualities and are usually not coated, however, some customers may want to recoat objects with worn factory applied lacquer coatings. They must first remove any remaining finish with lacquer removers, steel wool and metal polish. Applying a protective coating of lacquer will allow the original metal to show through, create a luster and protect it against abrasions. Should a customer want to paint over these metals, (heaven forbid) they should use a moisture-barrier alkyd primer and any compatible finish paint.

SHINGLE & SHAKE PAINTS

Used primarily for decorating wood surfaces, shingle paints are more like pigmented stains, leaving only a trace of color. Application to bare or previously painted wood needs no primer, although should be recommended if the wood is badly weathered.

Most shingle paints are oil - alkyd based. Shingles and shakes are usually coated with a clear stain or water repellent at the factory, both of which contain wax. No coating resin will incorporate the wax as they dissolve and most latexes will not bond with wax. Although some coating manufacturers have created specialized latex products just for shingles, you will have to survey the shelves to find specific products for painting shingles and shakes.

Wood shingles treated with creosote should not be painted for eight years. The oily creosote prevents the paint from bonding to the wood and can bleed through to discolor the new paint film. Note: creosote has been banned and is no longer available.

As an alternative to shingle paint, a water repellent wood preservative is a good suggestion. Also, aluminum paint and shellac adhere to creosote-treated wood.

PORCH & DECK PAINTS

Coatings for exterior walk-on surfaces go by many different names: Exterior floor paint; porch floor enamel; porch and deck paint and deck enamel. There are alkyd, oil, latex and urethane formulas for wood and concrete and some products even perform well on metal surfaces. They all are designed to be highly resistant to abrasion and weather.

Most solvent-based floor coatings provide a medium to high gloss while the acrylic latex products come in a wide range of glosses from flat to semi-gloss.

Acrylic latex floor coatings can be used on bare or previously painted wood floors. They are easy to clean up and the homeowner can later lay resilient floor tile on top of the old paint, which could not be possible with most other floor paints.

MARINE PAINTS (aka Spar Varnish)

Although somewhat expensive and originally designed specifically for boat hulls and decks, marine paints are increasingly popular for outdoor wood and metal house trim . They contain considerably more and higher quality resin than ordinary paints. The higher resin content accounts for its increased durability and excellent water and abrasion resistance . Mostly solvent-based, marine paints provide a glossy finish and adhere well to fiberglass.

ANTI- FOUUNG PAINTS

These are truly special purpose coatings- so specialized that many stores do not carry them unless they are located near a body of water. Formulated especially for boats, docks and other below-the-water-line surfaces they prevent "foul" or marine plants and animals such as barnacles, the hard shelled marine animals, from attaching themselves to boat hulls and slowing them down. These creatures can cut through ordinary protective coatings, exposing boat metals to corrosion.

Anti-fouling paints contain small amounts of chemicals such as copper, tin or bermanium, which are toxic to barnacles and other marine growth. These chemicals are gradually released from the paint film and kill any "fouling" plants and animals attached to the film.

Since these paints are toxic, strict environmental rules govern their manufacture, sale and use in many parts of the country.

PORCELAIN PAINTS

Most porcelain paints are clear coatings with very little solvent and a high concentration of epoxy resins. Usually these two-component products are combined right before applying.

Porcelain paints are used to restore bathroom fixtures, repair nicks, chips and scratches, or to change the color of the fixtures. They can provide a mar-proof,

water-resistant and abrasion-resistant surface to other materials.

Preparing the surface is critical when using porcelain paint. Soap residue must be removed completely and the surface must be very dry. Work must be done quickly since the two components, once brought together, immediately begin to cure.

ACOUSTICAL PAINTS

Designed especially for acoustical ceiling tiles, it forms a porous film which will not harm the noise-reducing properties of acoustical tile.

Compared to ordinary latex wall paint, acoustical paint contains more water and pigment and less resin . It is the same consistency as regular wall paint and can be applied in the same manner.


Now that you are an expert in what to use for your painting project, let's talk about how to use those materials in our next segment of the Painting Primer.

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