Chuẩn bị bề mặt trước khi sơn phủ

Bước 1: Kiểm tra độ ẩm của bề mặt Đây là bước quan trọng đầu tiên và không thể thiếu. Độ ẩm của bề mặt tiêu chuẩn là dưới 5%, nếu độ ẩm quá cao phải dùng dụng cụ chuyên dụng để xử lý như lớp vữa ngăn ẩm đối với sàn bề tông. Nếu bạn không xử lý thì hơi nước ở dưới sàn sẽ thẩm thấu ngược lên và làm bong lớp sơn. Bước 2: Xử lý và sửa chữa bề mặt Bề mặt bị yếu, hư hỏng phải được xử lý kỹ trước khi tiến hành tạo nhám. Tùy bề mặt sơn chúng ta dùng các dụng cụ tạo nhám khác nhau như: đá mài, giấy nhám, máy mài công nghiệp.. Bước 3: Vệ sinh trước khi tiến hành sơn công nghiệp Trước khi sơn công nghiệp chúng ta phải vệ sinh sạch sẽ bề mặt. Hút hết tất cả bụi và mặt để tạo độ bám dính và độ bền cho sơn.

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Frosio 06 Surface preparation slide # 1

ê Laminations

ê Flame cut edges

ê Weld spatter

Pre-blasting preparation

involves the following activities

Notches minimum diameter: 30 mm

Inspected and approved before cleaning

See ISO 8501 “Visual assessment of surface cleanliness”

Trang 2

Remove using grinder

Undercuts exceeding classification ruling should be repaired by welding and grinding.

Sharp profile peaks to be smoothed using grinder

Weld Weld

Trang 3

A

B

Gas cut edge

A Remove by grinder or disc sander.

B Rolled steel sections normally have round edges Therefore can be left untreated.

A Remove visible spatter before blasting with grinder or chipping hammer.

grit-B For spatter not readily removed, remove using grinder/disc.

Grinding of sharp edges

• Rounding of a sharp edge

• Paint will not be able to

give adequate film

thickness over a sharp

edge

• Rounding the edge

followed by stripe coating

gives an even film

thickness

• This will improve the

lifetime of the paint

system

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Cross section of

painted steel with a sharp edge

• Paint applied with spray

• Forms inferior coating thickness around the edge

Cd-4932-21

Steel Paint

Cross section of painted steel construction

with well rounded edge

• Paint applied by spray

• Forms even thickness around the edge

Cd-4932-20

Steel Paint

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Grinding of sharp edges, welds etc.

Disc sander and disc grinder

Sander for removing mill

scale, paint and rust

Grinder for heavy grinding, such

as edges and weld beads

Steel support prior to

pre-blasting preparation

• Sharp edges

• Rust grade: A to B according to ISO 8501

Cd-4930-46

Trang 6

Disc grinding of a construction

Even a small construction may have a of sharp edges

Cd-4932-11

Edge rounded with disc grinder

A well rounded edge will ensure a sufficient paint film

over the entire construction

Cd-4932-12

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Steel support after blast-cleaning to Sa 2 ½

The sharp edges should have been rounded

prior to blast-cleaning

Cd-4930-47

Steel support after blast-cleaning to Sa 2 ½

The sharp edges have been rounded prior to blast-cleaning: Good !

Cd-4930-48

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Pre-blasting preparation

Grinding a small construction

• Mark areas where

grinding are required

• Remove weld spatters

• Grind the welds

• Grind the notches

• Important to carry out

the grinding to improve

the life time of the paint

system

Pre-treatment: Grind_construction1

Early corrosion on sharp edge and spot welds

• Paint has newly been applied

• Exposed to wind and weather for approximately 2 weeks

Cd-4930-78

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Insufficient pre-blasting preparation

• Notch

• Power tooling has been carried out

• Sharp edge not grinded

Cd-4932-16

Grinding of notch with rotating file

Easy access with a rotating file

Cd-4932-18

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Small weld spatters

• Even only one

small weld spatter

may be sufficient

to initiate corrosion

which with time

may lead to severe

Trang 12

Remaining weld beads after removal of supports

welded on to the structure

Trang 13

Remaining weld beads after removal of

supports welded on to the structure

Disc grinding of weld beads, sharp edges etc.

by means of a disc grinder.

Trang 14

Paint on a very poor weld Pinholes and holidays

visible after second stripe coating

• Neither re-welding nor grinding have been carried out

prior to application

• Remedial Actions: Reblasting, re-welding, grinding and

blast -cleaning: COSTLY

Cd-4930-41

Paint applied on a very poor weld.

Pinholes and holidays visible after application

• Neither re-welding nor grinding have been carried

out prior to application

• Remedial Actions: Reblasting, re-welding, grinding

and blast -cleaning: COSTLY

Cd-4930-40

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Weld with pinhole.

Visible through the paint system

• Will lead to initiation of corrosion

• Remedial actions: Remove the paint, re-weld and grind

Severe corrosion due to a

combination of several effects

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Early corrosion of welds in a

water ballast tank

• Rough weld seams

Should have been

grinded

• Probably in

combination with

poor stripe coating

• Mud can be seen,

particularly on the

bottom

Cd-4930-75

Inside a tank after pre-blasting

and paint application

Trang 17

Steel support after pre-blasting

Inside a tank after pre-blasting

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Spot welding leads to crevices

which are susceptible to corrosion

Cd-4930-36

Sharp edge an spot welds.

Early corrosion attack

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Spot welds after painting

• Paint is unable to fill

gap between the

parts welded together

• These area may be

Trang 20

thick layers of rust

Maintenance General surface treatment routine

Surface preparation includes removal of:

•Washing with detergent

•Other methods agreed upon

Surface cleanliness Important to remove all contaminants

Important to remove all contaminants

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Welding smoke and other contaminants have not

been removed prior to application

• Small steel units welded on to a deck construction

• Welding seams has been wire brushed

• Welding smoke will result in osmotic blistering

• Reasons may be:

– Oil leaking out of

equipment

– Equipment has been

stored with oil

– Lack of oil trap (Air from

compressor contains oil)

• Must be removed prior

to surface preparation

Trang 22

Degreasing by wiping the surface with

solvents and rags is not recommended.

Leads to spreading of a thin film of oil over a wider area

and increases risk of fire / explosion

Cleaning procedure

below and upwards

• Work systematically on all

Trang 23

Degreasing side bottom with emulsifying detergent to

remove oil, grease etc.

• Should always be carried out before blast-cleaning

• The detergents must be removed by ”Low pressure

water cleaning, LPWC” (Around 250 - 300 bar)

CD 4932 no 23

Removal of marine growth, salts and

loose paint by Low Pressure Washing

Trang 24

The water quality can be

influenced by several factors

“Dust” after drying Loss of adhesion

Water quality:

Potential risks from unknown sources

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Osmotic blistering

Osmotic blistering occurs when painthas been applied on a surface contaminated

with water soluble salts

Paint technology \ Osmosis1

CD-1183 no.90 CD-1183 no.89

Formation of blisters as a function of salt

concentration on substrate (1 of 2)

Salt: 0 mg/ m²

Film: 150 microns

Salt: 60 mg/ m² Film: 150 microns

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CD-1183 no.92 CD-1183 no.91

Formation of blisters as a function of salt

concentration on substrate (2 of 2)

Salt: 100 mg/ m²

Film: 150 microns

Salt: 200 mg/m² Film: 150 microns

Welding smoke is water soluble and

can only be removed by water

• Solvents will not

CD 0589 no 11

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Blisters close to weld, probably due to

welding smoke remaining on the the steel

• Welding smoke is resoluble in water and will

create osmotic blistering

• Galvanic difference between steel plate and

weld may aggravate the attack

CD 4934 no 79

Tools and methods must be selected acc to:

•Paint specification

•Area to be repaired

•Degree of damage on surface

•Existing paint system and pre-treatment

Trang 28

(Available with and without vacuum unit)

min/m²

Achieved standard

Blast cleaning

Power chiseling/wire brushing

Manual scraping / wire brushing

Power grinding

Manual scraping + power wire brushing

Power hammer / wire brushing

Power needle hammer / wire brushing

6 9 13 15 15 25 37

* Only time required for the pre-treatment itself is given

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• Surface tolerant Mastic

paints are recommended

on such substrates

CD 4932 no 50

The tool can be used as grinder and sander,

depending on the disc Here: Disc-sander

• Particularly used on

surfaces which can not

be blast-cleaned

• May be difficult to

reach narrow spaces

and inside corners

• Surface tolerant Mastic

paints are

recommended on such

Trang 30

• Rotating wire brushes

speed up the production

rate

• Risk of having a polished

surface

• A polished substrate will

give poor adhesion to the

paint system

Pre-treatment: Wirebrush1

Trang 31

Wire brushing by hand to achieve given standards

according to ISO 8501

• Production rate is slow: High cost

• The quality is not as good as blast-cleaning or disc grinding

• The surface tolerant Mastic paints are recommended on wire

brushed surfaces

CD 4932 no 40

Steel substrate treated by hand and

mechanical power tool cleaning equipment

Hand wire brush, approximately St 2

Mechanical wire brush

approximately St 3

CD 4932 no 47

Trang 32

Hand wire brushed surfaces

Surfaces treated approximately to the

given standards according to ISO 8501

CD 4932 no 41 CD 4932 no 42

Different

Power tool cleaning methods

• Reciprocating impact tools

• Rotary impact or scarifying tools

• Grinders or sanders

Available as:

environmental concerns

Trang 33

Equipment: Hand and power tool cleaning

Vibration white finger:

Rotary impact or scarifying tools

• Peening flaps (Roto-Peen)

- Creates a surface profile, 25 to 75 microns

• Rotary hammers - cutters

• Nylon non-woven abrasive wheels

Rotary impact tools is a good choice

for removing coatings

Equipment with rotating abrasive head

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Hand held rotary impact tool Roto - Peen

• Rotating abrasive head: Peening flaps (Roto-Peen)

• Creates a surface profile, 25 to 75 microns

• What about deep pits ?

Trang 36

• Blast cleaning is the best alternative.

• Use mechanical cleaning only when

blast cleaning is not possible

• Rotating wire brushes and disc-sanders

may give a polished surface

Selection of pre-treatment method

Mechanical cleaning vs blast cleaning

Correctly carried out, blast cleaning

gives an ideal surface for painting.

Blast cleaning is time saving compared with mechanical cleaning

Surface preparation Major advantages with blast cleaning

Trang 37

Pre-treatment Illustration of various blasting methods

• Does not remove

salts from the

substrate

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Dry blasting

Benefits and limitations

Advantages

• Surface remains dry

• Good anchor pattern

for paint

• No pre-rusting profile

Disadvantages

• Does not remove salt

• Does not remove oil

• Creates dust

humidity, reduced work space

Trang 40

Well equipped blast- cleaner

• Hood with visor

• Correct gloves for

Vacuum Blasting EDUCT-O-MATIC

• Equipment for small

repairs

• Several types of nozzles

can be used Makes it

possible to obtain all steel

Trang 41

Vakuum blåser dekk

3 - 8 HWA

Vacuum Blasting on Deck

Slurry and wet blasting

Benefits and limitations

Trang 42

Wet Blasting: Mixture of water and sand

Blast cleaning with Metallic or

Non-metallic abrasives to specified:

• Cleanliness acc to ISO 8501 - 1 or 2

• Roughness acc to ISO 8503

Surface preparation Metallic or non-metallic abrasives metallic abrasives.

Trang 43

• Chilled iron grit (angular)

• High-carbon cast steel grit and shot (angular or round)

• Low-cast cast steel snot (round)

• Cut steel wire (cylindrical)

Nickel refinery Coal furnace Fused aluminium oxide Iron furnace

Abrasives: Two main groups

1 Metallic abrasives

2 Mineral and slag abrasives

Metallic abrasives and Carborundum and Al.oxide) can

be recycled and are usually used more than once.

For stationary plants

Mineral and slag abrasives are generally

used only once.

For open blast-cleaning

Metallic / mineral abrasives

Re-use

Trang 44

Abrasives and Blast Profiles

Abrasives Initial particle shape

Designation and initial

Grit - angular, irregular G

Cylindrical - sharp-edged C

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Type Abbreviation Initial particle shape Comparator 1)

Cast iron Chilled M/CI G G

Cast steel High-carbon M/HCS S or G S 2)

Cut steel

wire

Metallic (M) blast-cleaning abrasives

1) Comparator to be used when assessing the resultant surface profile The method

for evaluating the surface profile by comparator is described in ISO 8503-2.

2) Certain types of abrasive rapidly change their shape when used As soon as this

happens, the appearance of the surface profile changes and becomes closer to that

of the shot comparator

ISO 8504-2:1992 (E)Non-metallic, natural abrasives

Type Abbreviation Initial particle

shape Comparator

Silica sand N/Si

Olivine sand N/OL G G

Starolite N/ST S G

Garnet N/GA G G

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Non-metallic (N) synthetic abrasives.

Iron furnace slag (Calcium silicate slags) N/FE

Copper refinery slag (Ferrous silicate slags) N/CU

Coal furnace slag (Aluminium silicate slags) N/CS

Check with water

Abrasives Important points to check

Non - metallic abrasives

Trang 47

• Productivity is directly proportional to:

Pressure at Nozzle Capacity of the air compressor

• Pressure at Nozzle 7 kg/cm² = 100% productivity

• Pressure at Nozzle 5,6 kg/cm² = 66% productivity

• Pressure at Nozzle 4,2 kg/cm² = 50% productivity

Blast cleaning Factors influencing the production rate

Factors influencing the production rate

Air

Blast - cleaning Measure the air pressure at the nozzle

Pressure gauge

Trang 48

The blast hose shall have an opening

which is 3-4 times bigger than the

orifice of the nozzle

Blast Cleaning

Rule of thumb: To avoid loss of pressure

Hose opening Nozzle opening

3 - 4 times

Standard nozzle abrasive speed: 300 Km/h at 7 bar

.

Venturi nozzle abrasive speed: 700 km/h at 7 bar

Blast cleaning: Venturi nozzles increase

the speed of the abrasives

.

Trang 49

Corroded area Subsequent spot blasting

Solid coating

Feathered required Impact by abrasives

(SOURCE: Munger, C.G Practical aspects of Coating Repair.

Materials Performance, Vol 19, No 2 p 46 (1980)

Abrasives will damage the coating

Cracks due to direct

Corroded and blast cleaned

Area with

reduced adhesion

Steel

Area require feathering

Trang 50

Loose edges resulting from spot-blasting

carried out some time ago

• Edges have not been feathered prior to application of the paint

• The edges are weak points in the paint film

• Corrosion attack initiates on such areas

• Spot blasted hull

• Thick old paint

Tiêu đề	Surface Preparation Before Coating
Trường học	Frosio
Chuyên ngành	Surface Preparation
Thể loại	Course Handout

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Số trang	80
Dung lượng	8,78 MB