34 D+D OCTOBER 2014

Good Technical Practice

hop- or factory-applied, high-

performance coatings on stand-

ing seam roof panels, curtain

walls and many other architec-

tural metal surfaces can resist

weathering, abrasion, chemicals

and more, and can last well

beyond a decade. But even the

toughest coating eventually

needs repainting.

Unfortunately, some of the

same physical characteristics

such as high-gloss, smoothness and high

surface tension that give factory finishes

enhanced performance can also cause

them to resist field-overcoating, resulting

in an increased probability of premature

failure. Owners, designers and applica-

tors should understand the challenges

and risks involved before overcoating

factory-finished metal components.

SCoatings and ComponentsFactory-finished metal is usually made of

thin-gauge galvanized sheet steel or ex-

truded or thin-gauge rolled aluminum. Man-

ufacturers often apply finishes directly to

sheet metal before finish-forming is com-

plete in an automated “coil coating”

process. In some instances, however, man-

ual spray application is performed after the

components are formed.

These factory-coating systems are formu-

lated for superior physical performance.

They can maintain color, gloss and perform-

ance qualities, such as corrosion resistance,

for years. In fact, many building material

suppliers provide long-term warranties for

the finishes on their products. Some stand-

ing seam roof panel manufacturers warrant

the performance of factory-applied finishes

for up to 15 years against significant fading

or color shift, peeling and loss of gloss.

Making It

StickOvercoating factory-finished architectural metal poses problems,but proper prep can help.

James D. Machen, KTA-Tator, Inc.

fluoropolymers, polyesters, silicone-modified

polyesters and acrylics. When designers,

manufacturers or owners want superior

coating performance, fluoropolymer coat-

ings — formulations of 50 to 70 percent

PVDF (polyvinylidene fluoride) — are a

typical choice.

Factory baking is often required for

these coatings to achieve their maximum

performance capabilities. Post-application

baking further enhances the coatings’

physical properties, giving them excellent

color, gloss retention, durability and hard-

ness — often the very qualities that make

successful overcoating a challenge.

Five Steps for EvaluationSuccessfully overcoating a factory finish

starts with systematically evaluating the

physical characteristics of the existing coat-

ing. Steps 1 through 3 of the five-step eval-

Good Technical Practice35

Reasons for RecoatingHowever, even the toughest coatings may

eventually show the effects of Mother Na-

ture. Those effects can include fading, loss

of gloss from ultraviolet light and acid rain,

surface abrasion from ice and snow,

freeze/thaw cycling, and surface staining

from airborne dirt, grime and more.

While routine maintenance, such as an-

nual chemical cleaning, can prolong the

service life of factory finishes, eventually

repainting becomes necessary. And deteri-

oration is not always the sole force driving

the decision. New building owners or ten-

ants, new company color schemes, merg-

ers and acquisitions — all these and more

can be reasons to overcoat factory finishes.

Factory Applied Coating TypesCommon factory-applied finishes used on

galvanized and aluminum surfaces include

uation process categorize the risk of prema-

ture coating failure as high, moderate or

low, so you can make an informed decision

as to whether overcoating is viable.

These steps can also tell you whether

the surface being considered for overcoat-

ing is the original factory finish or if other

coating layers have been applied. That in-

formation is important for selecting com-

patible products and procedures for

recoating. Once you’ve confirmed that the

existing factory finish is a viable overcoat

candidate, step 4 helps you identify prod-

ucts and procedures.

(Facing page) The deteriorated factory finish andcorrosion make this metal roof an immediate can-didate for field overcoating, including significantsurface preparation.(Below) Factory-applied finishes such as the one onthis standing seam roof can maintain color, glossand performance qualities, such as corrosion re-sistance, for years. Photos courtesy of KTA-Tator.

The original construction warranty docu-

ments are another information source.

They can identify the material supplier,

manufacturer, factory finish trade names

or even the shop applicator. Manufacturers

or applicators may be able to provide infor-

mation on the products since they often

use single-source suppliers for their fac-

tory finishes.

If project records are inconclusive, labo-

ratory infrared spectroscopic analysis of

samples can help identify coating types.

Step 2 – Visually Assess

Visually assess the factory finish for corro-

sion and paint degradation, such as lifting

or peeling paint. Visual reference stan-

dards such as SSPC-VIS 2 Standard

Method for Evaluating Degree of Rusting

on Painted Steel Surfaces, can help charac-

terize the type and quantity of rust.

Also check for surface cleanliness. Con-

taminants, corrosion and peeling paint can

interfere with overcoat adhesion. This

may seem like an obvious step, but it’s re-

markable how often it’s overlooked, result-

ing in premature failure of the re-coating.

Step 3 – Test for Dry Film

Thickness and Adhesion

Dry film thickness— In general, as coating

thickness increases so does the risk asso-

ciated with overcoating. The weight of the

36 D+D OCTOBER 2014

Step 5 — candidate coatings test

patches — tests and validates the suitabil-

ity of the surface preparation methods and

coating materials selected.

Let’s take a closer look at these five

steps.

Step 1 – Determine the

Existing Finish Coating Type

The generic identity of the existing factory

finish, or if possible, the actual coating

products applied, must be determined to

ensure compatibility with candidate over-

coating materials. The original project

specification or construction procurement

documents often provide specific informa-

tion concerning the type of factory finish

that was required. While this information is

valuable, it might not reflect substitutions.

(Below) The high-performance features of bakedfinishes, like the ones on the metal roof and fix-tures of this commercial building, can make over-coating difficult. Relatively recent advances inair-drying fluoropolymers and water-based acrylicbonding primers have helped to address the issue.

Step 4 – Choosing Materials

for Field Overcoating

If the preceding steps indicate that the

existing finish is suitable for overcoating,

then the next step is choosing a coating.

The candidate materials must be compat-

ible with the existing factory finish;

amenable to field application; must ad-

here to field-prepared factory finishes;

and must have the physical characteris-

tics, such as weatherability and color and

gloss retention, needed to perform in the

exposure environment.

Choosing an inappropriate overcoat

material can lead to premature coating

problems or a decrease in service life. We

must also recognize that field-applied

overcoats typically do not provide the

same “new car” appearance as factory-

applied finishes. Qualified, experienced

field applicators, however, can certainly

produce high-quality, aesthetically-pleas-

ing finishes.

coating with the knife point at the vertex

of the angle. The result is rated on a scale

included in ASTM D6677.

The pull-off adhesion test— This method

uses an adhesive to attach an aluminum

test fixture to the surface of the factory fin-

ish. After the adhesive cures, a test instru-

ment is attached to the fixture and the

fixture is pulled from the surface. The pull-

off or tensile strength of the coating is re-

ported in pounds per square inch (PSI). In

addition, the side of the test lug that was

adhered to the coating is inspected to deter-

mine the type and location of adhesive

break at the substrate or between layers, or

a cohesive break within layers. The location

of fracturing typically indicates the “weak-

est link” in the coating system.

Guidance concerning characteristics that

existing coatings must have to be consid-

ered low-risk candidates for overcoating

can typically be obtained from industry

publications and coating manufacturers.

Good Technical Practice37

coating film, and the contractive stress as

added coating layers dry and cure, puts

stress on the underlying layers. Eventually

those stresses can exceed the adhesive or

cohesive strength of the existing coating,

and problems such as cracking and peeling

can result.

Measure the total thickness of the paint

film at enough locations to determine a

representative thickness. Record the

ranges and average the measurements.

This is commonly done with magnetic-

type, dry-film thickness gages.

Adhesion— Determine the adhesion of

the factory finish to learn whether or not

there is a sound foundation over which

new field overcoats can be applied. If adhe-

sion is insufficient, the weight and stress

from contractive shrinkage as the new

coating dries and cures may cause prema-

ture cracking or peeling.

Field-coating adhesion is typically meas-

ured in accordance with ASTM D3359

Measuring Adhesion by Tape Test; ASTM

D6677 Standard Test Method for Evaluat-

ing Adhesion by Knife Test; or ASTM

D4541 Standard Test Method for Pull-Off

Strength of Coatings Using Portable Adhe-

sion Testers.

The tape test — This test scribes evenly

spaced lines in a lattice pattern through

the coating film to the substrate. Apply a

pressure-sensitive tape over the scribed

pattern, then remove it. Rate adhesion by

comparing the amount of coating removed

along the lattice pattern with written de-

scriptions and illustrations within the stan-

dard and progressive rating from 0B to 5B.

The low end of the scale, a 0B rating, indi-

cates poorest adhesion with the most coat-

ing removed, while the high end of the

scale is the 5B rating — no coating re-

moved — indicating the best adhesion.

The knife test— The knife test makes an

“X” cut with a razor knife. Try to lift the

coating from the substrate or underlying

While field-applied overcoats can approximate factory finishes, they typically do not provide the same“new car” appearance. Here, a technician takes a field gloss-measurement on coated sheet metal roof.

38 D+D OCTOBER 2014

Compatibility with Existing

Factory Finishes

The best candidate overcoat materials are

typically those with a proven track record

of success. Unfortunately, many factory-

applied coatings are specialized formula-

tions, requiring post-baking to cure, and

can’t be field-applied.

To address this, manufacturers have de-

veloped air-drying fluoropolymer formula-

tions. These high-performance field-

applied coatings are compatible with

baked fluoropolymer coatings.

Many prominent industrial and architec-

tural coating manufacturers have also devel-

oped waterborne acrylic bonding primers for

application over challenging surfaces such

as factory finishes and other smooth, hard

or glossy surfaces or substrates.

If manufacturers of such materials know

the generic type of factory finish to be

overcoated, they can recommend products

and procedures.

The manufacturer of the factory-applied

finish, if known, can also provide insight

into candidate overcoat materials.

Unfortunately, on some projects, infor-

mation concerning the factory-applied

coating remains a mystery. In that case,

apply a field test patch. In fact, applying a

field test patch in advance of any overcoat-

ing, mystery or not, is a best practice.

Step 5 – Field Test

Before overcoating, even with products

recommended by the manufacturer of the

factory finish, install a test patch to con-

firm that the system will perform.

Test patches help you assess surface

cleanliness; the means and methods re-

quired to achieve that cleanliness; and

what negative effects, if any, that the sur-

face-preparation process may have on the

substrate; and other physical characteris-

tics including appearance, color, gloss and

adhesion of candidate coating materials

selected in step 4.

The test patch design can also include

multiple levels of surface cleanliness to

evaluate the minimum level of surface

preparation needed. Test patches also let

you try multiple candidate coating systems

to select the best-performing materials.

The in-service exposure time and the in-

terval between evaluations of the test

patch are also important. In general, the

longer the test patch is exposed, the more

useful the information is for highlighting

risk and predicting service life. At a mini-

mum, meet the manufacturers’ recom-

mended curing times before evaluating the

test patch. Coatings typically do not maxi-

mize their performance properties until

fully cured.

ASTM D5064 Standard Practice for Con-

ducting a Patch Test to Assess Coating

Compatibility offers a recognized, ac-

cepted protocol for applying and evaluat-

ing coatings test patches.

Preparing the Factory Finish for OvercoatingRemove loose or deteriorated coating and

corrosion by cleaning in accordance with

SSPC-SP 2 Hand Tool Cleaning, and/or

SSPC-SP 3 Power Tool Cleaning. Feather

the edges of the coating around repair

areas, commonly done with fine sandpa-

per, to achieve a smooth, tapered transi-

tion from the repair area to the

surrounding intact coating.

In addition, the exposed edge of the

coating around the repair areas should be

tested for adhesion. Adhesion is adequate

when the coating edge cannot be lifted by

manual probing with a dull putty knife.

The preceding SSPC standards, SSPC-SP 2

and SP 3, describe the dull putty knife

procedure.

Cleaning — Remove dirt, grime, grease,

oil, chalked coatings and other contami-

nants by pressurized water-cleaning.

Water pressures of 3,500 to 5,000 PSI are

A worker uses fine sandpaper to superficially roughen a factory finish on a sheet metal roof before over-coating. This breaks the surface tension, increases the surface area and provides “tooth” for mechanicaladhesion of subsequent coatings.

PPG pmc in Quark_Layout 1 8/4/14 11:41 AM Page 1

40 D+D OCTOBER 2014

typically effective for this purpose; how-

ever, higher pressures are sometimes

necessary.

The fan angle at which the pressurized

water exits the nozzle tip can affect the

quality and rate of cleaning. Nozzles with

zero-degree rotary tips and 15- to 25-de-

gree tips are common, but be aware that

tips with angles of zero to 15 degrees can

be aggressive enough to damage the coat-

ing film or the galvanized or aluminum

substrate if held too long or too close to

the surface.

Adding commercial detergents and

scrubbing with stiff bristle brushes can

help remove stubborn contaminants.

Mildew and other fungal growths are typi-

cally eliminated with bleach or bleach/deter-

gent blends followed by thorough rinsing.

Roughening — Factory finishes, typically

applied at just a few mils, are thinner than

conventional field-applied coating systems,

which usually go on at about 10 mils or

more. Take care to roughen the surface

superficially without removing or damaging

underlying coatings or the metal substrate.

Many coating manufacturers suggest

that the surface roughening is adequate

when the gloss of an existing coating film

is completely removed or dulled. Gloss re-

moval and surface roughening is typically

accomplished by hand- and power-sanding

or by carefully controlled brush-off blast

cleaning.

When sanding, it’s best to use a finely

graded sandpaper that roughens the fac-

tory finish only superficially without dam-

aging the metal or coating layers beneath.

Brush-off blast cleaning requires ex-

treme care because it is easy to create

problems for overcoating by fracturing and

weakening the existing film or damaging

the substrate. Control measures include

reducing compressed air pressures, using

a fine abrasive, and increasing the blast

nozzle to work-piece distance.

In that regard, SSPC-SP 16 Brush-Off

Blast Cleaning of Coated and Uncoated

Galvanized Steel, Stainless Steels, and

Non-Ferrous Metals provides useful guid-

ance for blast cleaning coatings on galva-

nized steel or aluminum for overcoating.

Less-aggressive abrasive blast media

has also proven effective for cleaning and

roughening without unnecessary damage.

Methods include using blast media such as

urethane foam sponge impregnated with

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Good Technical Practice41

abrasive; soft abrasive media such as wal-

nut shells, crushed glass or other hybrid

abrasive mixtures; or newer low-impact

blast technologies utilizing water vapor at

low operating pressures of around 40 PSI.

Traditional blast-cleaning pressures of

90 to 100 PSI or more are excessive for this

application.

Regardless of the method used, surface

roughening should produce a dense, uni-

form surface profile in a continuous pat-

tern of peaks and valleys, leaving no

smooth, un-profiled areas.

The final step is ensuring the surface of

the factory finish is dry and that dust or de-

bris from surface preparation is removed.

Keys to SuccessOvercoating factory finishes can be chal-

lenging but you can overcome those chal-

lenges with proper planning. Key steps

include:

(1) determining the generic coating type

of the existing finish,

(2) visually assessing the physical char-

acteristics of the existing finish,

(3) mechanically testing dry film thick-

ness and adhesion,

(4) reviewing candidate overcoat mate-

rial, and

(5) incorporating steps 1 through 4 into a

coatings test patch program so that you

can make an informed decision.

While these steps require money, time

and work, they help prevent costly sur-

prises during overcoating, and ultimately,

contribute to successful projects.

About the AuthorJim Machen is a senior coatings consultant

with KTA where he has worked for more

than 20 years. He is a NACE certified coat-

ings inspector Level 3 (peer review), an

SSPC certified protective coatings special-

ist, and is a Level II inspector in accor-

dance with ASTM D4537. Jim consults on

coating failure

analysis, coating

system recom-

mendations, spec-

ification

preparation, and

major project management for clients in

the transportation, water and waste,

power, chemical and marine industries.

D+D

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