Page 431 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/405
The size, however. should be commensurate with the it produces acurtain ofwater todissolve all oversprayed
size of the assemblies and the scale of work. paint. The dissolved paint can then be collected at the
bottom in a trough and disposed of. and the trough-
Automation contaminated water can be drained out after neutra-
lization and effluent treatment. Figure A 13. l illustrates
The entire sheet pre-treatment process described above a typical layout of a medium-sized paint shop.
can also be made automatic as noted below: Electrostatic method This is also a wet method like
the conventional process, except that the paint is now
Set each thermostat at the required temperature when- electrostatically charged, similar to the powder paint
ever heaters are provided. in a dry method as discussed later. The paint, being
Define the process time of each operation and set the highly charged electrostatically, is wrapped around the
hoist to dip. lift and carry the job to the next tank etc. object automatically.
Liquid paint
A13.5 Procedure for liquid painting
These paints are resin based and the paints required for
sheet-metal surfaces are generally alkyd-based resins.
Making the surface For general industrial applications. any of the following
Within 12 hours of surface pre-treatment the surface must types of enamel paints may be used,
be realed through a coat of primer as described in Table
A13.1. After the primer coat, the surface must be air Air drying
dried or stoved. The stoving method is always preferred, Air drying-cum-stoving
being faster and neater, compared to an air-drying process, Stoving
which takes longer to set, and the painted surfaces may
collect suspended dust particles from the atmosphere.
After the primer is set. the surface may be applied, if For special applications, however, such as for normally
required, with a very thin coat of putty to fill in any pin humid areas, and contaminated or chemically aggressive
holes or other irregularities. The putty is also air dried or locations, epoxy paints ai-e considered to be more
stoved and then rubbed gently with emery paper and appropriate. They provide a protective coating which is
washed with water to obtain a smooth plane surface, resistant to chemical fumes, corrosion and temperature.
ready to be coated with the final paint. In fact, putty Chlorinated rubber paints, which also fall into the same
filling is not recommended because it is wasteful. It also category of prolective paints, may also be used for these
adversely affects the strength of the paint film and areas but, not being temperature resistant. are not preferred
to epoxy paints.
increases porosity on the surface, and should be avoided Preparation of paint, its viscosity, solvent, thickness
;IS far as possible. It may not be necessary when cold- of one coat, air pressure, curing temperature and time of
rolled sheets are used for fabrication.
curing will remain the same as for the primer (Table
A13.4).
A13.6 Applying the final coat of paint
Important notes on Table A13.4 and procedure for
painting
After the surface has been prepared, the final coat of
paint is applied. The brief procedure for painting is almost I Thickness of coat:
the same as for the primcr and described in Table AI 3.4. The recommended thickness of the total coat (primer
It is recommended to apply the primer or paint inside plus paint) will depend upon the site conditions.
a spray booth. which would offer the following advantages: For a normally clean environment, a coat of up to
50 microns is considered adequate. For a dusty or
Conventional method using a spray booth (wet humid location requiring constant servicing and
method) This traps the primer and the paint fumes, cleaning, a thicker coat, say, up to 70-80 microns.
after routing them through a curtain of water, and then is considered to be adequate.
exhausts them into the atmosphere. This procedure, A thickness of up to SO microns is possible through
therefore, causes no environmental pollulion. Within one coat of primer and paint. To obtain a greater
the plant area it also protects the operator and others thickness an additional coat of paint. rather than
from a health hazard. It also protects machinery installed primer, may be applied after almost curing the first
nearby from paint fumes and also the plant from a fire coat. A thickness of primer of more than 30 microns
hazard. The waste water, after treatment and is riot considered satisfactory as it may diminish its
neutralization, is discharged into the drains. To achieve adhesive properties. To obtain a thickness of up to
this, the spray booth is provided with blowers on the top 100 microns, for instance, each coat (one of primer
having its suction through a trough of water. (Refer to and two of paint) may be around 30-35 microns.
Figure A 13.1, illustrating this arrangement.) It serves a Whenever a second coat of paint is required for
dual purpose: first. it creates a draught of air within the better adhesion of paint, it is better to rub the painted
booth to help eliminate all the paint fumes and second, surface of the first coat with a finer emery paper