I.Introduction to Finishing

Finishing

I.Introduction to finishing

A.Definition of finishes

B.Types of Finishes

a.Mechanical

i.Involve the use of no chemical or electrochemical treatment and no additive

ii.Provide a surface texture by mechanical means

iii.Range greatly from mill-finish to polishing

iv.Includes both directional, non-directional and patterned finishes

b.Chemical

i.Processed by washing or dipping fabricated product in chemical solutions

ii.Can be an intermediate process to clean without changing appearance

iii.Can give matte textured appearance or as an intermediate finish

iv.Can chemically prepare the metal for later finishing.

v.Can be a final finish with a patina or oxide

c.Coatings

i.Applied to the Surface of fabricated product

ii.Many different types and application methods.

C.Importance of selecting the right finish

II.Types of Finishes

A.On Aluminum

1.Mechanical

a.As Fabricated

i.Also known As Mill finish

ii.Subsequent fabrication operations (forming, sawing, welding, punching, etc) can mar the appearance.

iii.Hot Rolled and heat-treated products typically have a dull and darker surface

iv.Cold-rolled has a brighter and more metallic color, Extruded is similar to cold-rolled but has striations caused by the extrusion die.

v.Castings have a rougher matte finish, die castings being smoothest, and sand castings the roughest.

b.Buffed

i.Created by successive processes of grinding, polishing, and buffing.

ii.Top Quality bright appearance, commonly used for entrance doors and framing.

iii.Should NOT be used on wide flat surfaces because of their reflectivity

iv.Smooth Specular is the brightest mechanical finish obtainable and is produced by buffing. Material is ground and polished to a lustrous finish, final stage is buffing with 320 grit.

v.Specular is the finish created by buffing only, with no preliminary grinding or polishing, the resulting finish can show some evidence of surface irregularities

c.Directional Textured Finishes

i.Smooth Satiny sheen, but limited reflectivity, caused by characteristic tiny parallel scratches produced by wheel or belt polishing with fine abrasives, by hand rubbing with stainless wool , or by brushing with abrasive wheels.

ii.Fineness of the finish is controlled by the size of grit used, speed of the belt, hardness of the block or pad used, the pressure exerted on the block and the condition of the belt.

iii.Fine satin, Medium Satin, Coarse Satin: Produced by wheel or belt polishing with grits of varying degrees of fineness

iv.Hand rubbed: produced by rubbing with abrasive cloth or stainless steel wool o increasing degrees of fineness, finishing with No. 0 or No.00. Expensive, only used where the nature of the work being finished makes other methods impractical or to blend in other finished.

v.Brushed: Produced by using power driven stainless steel wire wheel brushes, brush backed sander heads, abrasive impregnated foamed nylon discs or abrasive cloth wheels.

d.Non-Directional

i.Matte finishes of varying degrees of roughness produced by blasting a media (such as sand, aluminum oxide, glass beads or other specialized media) against the metal under constant controlled conditions

ii.Texture of the surface is determined by the size and type of abrasive media used, air pressure and position and movement of the nozzle

1.The Finer the abrasive the smoother the finish.

2.Silica sand is coarse, dust blasting with a very fine abrasive or ‘vapor honing’ with a slurry of extremely fine abrasive and water produces the finest finishes

iii.Not recommended on material less than ¼” thick.

iv.Can have difficulty in maintaining a uniform appearance without special equipment

v.Even the finest finishes are rough, show fingerprints easily, and collects and retains dirt

1.Some type of protective treatment is necessary to retain a clear appearance, such as clear lacquers or anodizing

vi.Common variations obtained from washed silica sand or aluminum oxide:

1.Extra fine matte, Fine Matte, Medium Matte, Coarse Matte

vii.Shot blasting is used to provide peened finishes, using steel shot of different sizes. Can produce the following finishes:

1.Fine shot blast, Medium shot blast, Coarse Shot Blast

e.Patterned

i.Available in thin sheet

1.Created by passing the sheet between two machined matching design rolls, aka embossing; or between a solid roll and a design roll aka coining.

2.Chemical

a.Non-Etch Cleaning

i.Degreasing

1.Vapor Degreasing, done in special degreasing machines

2.Solvent or solvent emulsion cleaning with hydrocarbon solvents to remove oil and grease from the metal surface

3.Should be followed by chemical cleaning

4.Can be subject to EPA regulations

ii.Chemical Cleaning

1.Method used when animal fats and oils must be removed

2.Metal is sprayed or immersed in any number of proprietary or non-proprietary solutions, usually at elevated temperatures, then rinsed with water.

3.Inhibited chemicals should be used on aluminum to prevent etching of the surface.

b.Matte Finishes

i.Referred to as Etched finishes or ‘frosted’ finishes

ii.Can be used to prepare for anodizing, or can simply be followed by the application of a clear lacquer

iii.Change the surface texture of the metal from a metallic shine to a stain or dull sheen and are regulated to give varying degrees of roughness.

1.Done using various solutions of acid or alkali and sometimes additives

2.For Production quantities, this can be cheaper than mechanical means

iv.Wide variety of acid and alkaline etches are used

v.Bright Finishes

1.Range from Mirror bright to diffuse bright

2.Chemical brightening, obtained by immersing the metal in certain acid solutions or Electrolytic brightening

3.Electro-polishing

a.Minute irregularities in the metal surface are removed by making the piece to be brightened the anode in an electrolyte

b.Produces pieces free from mechanical stresses and embedded abrasive particles

c.Different appearance than mechanical buffing.

4.Highly Specular: This finish is the result of preliminary buffing, followed by electro-polishing or chemical brightening and produces a mirror like surface on certain alloys, can be used for reflectors

5.Diffuse Bright: A finish generally produced by first applying a caustic etch (Medium matte) followed by chemical brightening.

c.Conversion Coatings

i.Generally used for preparing the surface for paint

1.However, can be used as final finish

ii.Natural oxide layer on aluminum does not provide a good bond for paints, or other coatings, it can be ‘converted’ to improve adhesion.

3.Anodic Coatings

a.Consists of immersing the aluminum to be anodized in an appropriate acid solution, referred to as the electrolyte and passing a direct electric current between the aluminum and the electrolyte, with the aluminum acting as the anode.

b.Results in the controlled formation of a durable oxide layer or coating on the surface of the aluminum.

i.Coating is many time thicker than naturally formed oxide film

ii.Can be transparent, translucent or opaque, depending on the alloys and/or processes used

iii.This coating does not affect the surface texture of the aluminum, however fine this texture is, but they greatly increase resistance to corrosion and provide increased resistance to abrasion

c.Proper surface pretreatment

i.Chemical cleaning and finishing is paramount

1.Degreasing or inhibited chemical cleaner is the first requirement

ii.Metal residue from finishing or grinding must be removed

iii.Chemical etching or brightening can be used following the cleaning where the final appearance requires such pre-anodizing treatments.

iv.Can receive one of the buffed, directional, non-direction or patterned finishes.

1.However As-Fabricated finishes are of a quality that eliminates the need for subsequent mechanical finishing operation

2.Most architectural products today are simply etched and anodized.

d.Several different anodizing processes are presently used for architectural aluminum products. They differ in solution used for electrolyte, voltages and current densities required, and the temperature at which the electrolyte is maintained.

e.For exterior applications thickness should be at least .7 mil and minimum coating weight of 27mg per square inch.

i.For resistance to extreme exposure conditions such as sea water or abrasive actions, integral color anodic coatings or ‘hardcoats’ up to 3.0 mils thick.

f.Sealing of the pores in the oxide coatings is essential.

i.Most methods are sealed in a deionized boiling water bath or metal salt sealant.

ii.Surfactants and wetting agents can be used but should be controlled so as not to cause poor adhesion of any organic over-coating.

g.Architectural Class 1 Coatings

i..07 Mil or more in thickness and weighing not less than 27 mg per sq. in. or a density not less than 38 g per sq in.

ii.Appropriate for interior architectural items subject to normal wear, and for exterior items that receive a minimal amount of cleaning and maintenance.

iii.It is possible to produce hard coat Class 1 finishes in thicknesses ranging from 1 to 3 mils.

h.Architectural Class 2 Coatings

i.Thickness ranging from .4 to .7 mils with corresponding weights of from 15.5 to 27 mg per sq. in. or a density ranging from 22 to 38 g per sq in. The lower figures being the minimums recommended for architectural uses

ii.Suitable for interior items not subject to excessive wear or abrasion or exterior items such as storefronts and entrances which are regularly cleaned and maintained.

4.Finish Considerations for Aluminum

a.See Cleaning and Maintenance Guide for Architecturally Finished Aluminum AAMA 609/610-02

b.Cleaning

i.Cleaning should begin as soon after installation as possible

ii.Use water at moderate pressure, starting at the top working downwards.

iii.If heavier cleaning is needed scrubbing with brushes or sponges and a mild detergent can be used.

iv.Do NOT use aggressive alkaline or acid cleaners.

B.On Copper and Copper Alloys (OMITTED FOR BASIC)

1.Mechanical

2.Chemical

3.Concerns when finishing Copper and Copper Alloys

C.On Stainless Steel (OMITTED FOR BASIC)

a.Mechanical

b.Polished

c.Colored

d.Electopolished Finishes

e.Maintenance and Cleaning

D.On Carbon-Steel and Iron

a.Mechanical Surface Treatments

i.Mill Finish

1.Hot-rolled mill finish: Tight mill scale and rust powder

2.Cold-Rolled Mill finish: Should be properly degreased before painting, may need roughening to insure good adherence

ii.Cleaning

1.Hand Cleaning: wire brushes, abrasive paper or cloth, scrapers, chisels, or chipping hammers. Be used for spot cleaning

2.Power tool cleaning: power driven brushes, grinders and sanders, used for removing scale, rust and dirty from material. Less appropriate for thin metal

3.Shot-blasting and Sandblasting: Excellent for obtaining a clean surface suitable for painting may be done either wet or dry, use with caution on thin metal. Also ensures the surface is sufficiently roughened to ensure good paint adhesion.

b.Chemical Treatments

i.Cleaning Processes

1.Pickling: Removes scale and oxide coatings; consists of immersing the metal in a dilute acid solution followed by rinsing in water, then in other solutions and a thorough drying.

2.Vapor Degreasing: uses vapors from chlorinated solvents in special degreasing machines. Can be toxic and comparatively expensive.

3.Alkaline Cleaning: Sprayed with or immersed in any number of solutions, usually at elevated temperatures, after which it is rinsed with water, Caustic soda, soda ash and alkaline silicates and phosphates are common cleaning agents.

c.Conversion Coatings

i.Converts the chemical nature of the surface layer to improving the bond for applied coatings.

d.Applied Coatings

i.Almost all iron and steel products receive some type of applied coatings.

ii.Organic and metallic are most common, but also used are vitreous, and laminated coatings.

iii.Metallic Coatings

1.Common Metals include Aluminum, Nickel Cadmium, Terne metal, Chromium, Copper, Tin, Lead and Zinc

2.Protects against corrosion, permitting the mechanical properties of steel to be used with the assurance of durability.

3.Some Metallic coatings provide unique protective value through electrolytic action.

a.If the coating metal is less noble than the base metal, it protects by acting as a sacrificial metal

b.If the coating metal is more noble, it has its own relative chemical stability to protect the steel beneath.

c.Hot Dip

i.Commonly used for Zinc Galvanizing, but also used for tin, lead, and aluminum.

ii.Steel item is cleaned and immersed in molten metal

iii.Drainage points to allow molten zinc to flow into all cavities are essential, be sure to prevent puddling areas.

iv.Tubular fabrications and hollow structural must be properly vented. Good galvanizing covers the inside and out. After galvanizing vents and drain holes can be plugged with zinc plugs

v.Some assemblies may distort at galvanizing temperature.

vi.Talk to your galvanizer about special procedures including bracing, venting and differing alloys used in assemblies.

vii.Untreated galvanized surfaces generally do not bond well with paint.

1.Zinc dust-zinc oxide and cement-in-oil paints or specially designed paints can have good results.

2.Weathering can increase the adhesion of paints.

d.Thermal Spraying, AKA Metallizing.

i.Uses heat and pressurized air to melt the coating metal and spray it onto the steel item to be coated.

ii.Can be used in the field

iii.Coating metals include zinc, aluminum and copper.

e.Electroplating

i.Zinc and Cadmium are common anodic coatings. Nickel, Chromium, and copper and cathodic coatings.

f.Cladding

i.Produces metal products consisting of a steel core, covered with a thin layer of coating metal.

ii.Commonly Copper, Stainless, aluminum, or lead

g.Metal Heavy paints

i.Not technically a metallic coating

ii.Zinc rich paint commonly used.

4.The thicker the coating, the longer the rust-free life.

5.Zinc and Aluminum are the most common.

E.Applied Coatings

a.Coatings are the most common way of adding attractive colors, textures, or sheens to a surface.

b.Composition

i.Binders: Materials that form the continuous film that adheres to the substrate, or that minds together other substances in the coating to form a film.

ii.Volatile components: liquids that make the coating fluid enough for application, then evaporate during and after the application. Found in most coatings, these can be an environmental and safety concern.

iii.Pigments: insoluble solids that are dispersed in the coating that remain suspended in the bind after film formation. Pigments generally provide the color and opacity to a coating. Clear coats contain no pigments.

iv.Additives: materials that modify some property of the coating. Such as catalysts for polymerization reactions, stabilizers and flow modifiers.

c.Definitions

i.Enamel: type of paint distinguished for its gloss. Usually has a higher percentage of liquid binger to make them harder smoother and less porous.

ii.Lacquer: clear or colored solution coating that dries by solvent evaporation alone.

iii.Waterborne coatings or water-based paints: Save as conventional paints except that the resin (binder) has been suspended in water through the use of surfactants and that most of the solvents are replaced by water. These typically have lower VOCs than most pains and are more environmentally friendly.

iv.Powder-coatings: one-part coating, sprayed in powder form onto the product and adheres by electrostatic forces. This coating is then baked to cure and set finishing to a smooth coating.

d.Properties of Coatings

i.Flow: the ease with which a coating can be applied. Too much flow may result in runny, poor hiding film, too little flow can result in brush or roller drag, slowing application, increasing labor costs and creating imperfections.

ii.Leveling: the ability of a coating to smooth out after application. Coatings should spread evenly over the surface, without valleys, stipples, or orange peel effects.

iii.Film Thickness: Thickness is directly related to protection against moisture and corrosive environments as well as other potential sources of damage.

iv.Spreading Rate: depends on the desired film thickness and the amount of hiding pigments in the coating.

v.Permeability: The ability of moisture and gases to work their way through, this is diminished by increasing film thickness or multiple coatings

vi.Adhesion: How well the coating sticks to the substrate, this is vital to avoid cracking and peeling. Clean surfaces and surface profile are very important factors to improve adhesion.

vii.Flexibility: Essential if the coated material is to be subjected to forming, if the project involves long stretches, thermal expansion and contraction can cause stress and failing in inflexible coatings.

viii.Abrasion and impact resistance: minimizes physical damage to the film both between the application of successive coats and after the finish product is used.

ix.Stain Resistance: Important in public areas where graffiti can be an issue.

x.Weatherability: this trait refers to the ability of a metal coating to resist chalking due to UV rays, wind driven rain, salt spray and wind borne chemicals.

xi.Weldability: the ability of a coating to withstand the heat of welding without losing its protective value.

e.Film Formation-Drying and Curing

i.If solvents are used the coating dries through evaporation, this can happen at room temperature or with the aid of heat.