Page 432 - Industrial Power Engineering and Applications Handbook

13/406 Industrial Power Engineering and Applications Handbook
Table A13.4 Priming the treated surfaces

Description Air drying Air drying-cum-stoving Stoving
1 I 3
1 Purpose (i) To provide protection to the treated surfaces by sealing
(ii) To provide an adhesive surface for the final paint
(iii) To smoothc thc surface
2 Types of primers (i) For synthetic paints, zinc-chromate primers are recommended, which contain zinc chromate
pigments and are highly corrosion resistant
(ii) For epoxy paints, epoxy primers must be used
3 Preparation (i) Remove the skin from the surface of the primer, if any, and stir well the contents of the drum
to make it a homogenous mixture
(ii) Filter the mixture through a cambric cloth
4 To adjust the viscosity by a 2 1-25 seconds 2 1-25 seconds 21-25 seconds
Ford Cup Viscometer No. 4 (as for synthetic and epoxy primers)
~- ~
5 Solvent (thinner):
(i) For synthetic primers General purpose Stoving Stoving
(ii) For epoxy primers EPOXY EPOXY EPOXY
6 Thickness of one coat:
(i) Synthetic primers 25-30 microns 25-30 microns 25-30 microns
(ii) Epoxy primers 35-50 microns 35-50 microns 35-50 microns

- ~. ~ _ _
8 Curing temperature:
(i) For synthetic primers Room temperaturea (i) Room temperature" or T
120- 1 30"C(Maximum
(ii) 100-120°C 140°C)
(ii) For epoxy primers
(a) Air drying Room temperaturea (i) Room temperature" or Not recommendedb
(b) Epoxy ester - (ii) 60-80°C 1 40-1 50°C
9 Curing time
(i) For synthetic primers 2 hours surface dry, (i) 12-16 hours at room temperature 20-30 minutes
12-16 hours hard dry (ii) 20-30 minutes at 100-120°C
(ii) For epoxy primers
(a) Air drying 2 hours surface dry, (i) 12-16 hours at room temperature -
12-16 hours hard dry (ii) 20-30 minutes at 60-80°C
(b) Epoxy ester - ~ ~ 3040 minutes
10 Surface fillers (putty)' Air drying Air drying or stoving Stoving
< (synthetic or epoxy, as the case may be) 3
I1 Final surface making Rub the putty, if applied, with water and slightly coarse emery paper No. 180-220. It is better to rub
the surface, even when no putty is applied to provide the surface with a knurling effect, to have a
better adhesive surface for the paint.
a Room temperature is considered to be around 4045°C. In winter or for cold climates, where the room temperature may be less than this,
suitable heating arrangements must be provided to obtain the desired results.
Air-drying epoxy paints are not required to be stoved due to chemical reaction at about 12O"C, which may affect its hardness. To speed
up the curing, it may be stoved at a maximum temperature of, say, 60-80°C as noted in column 2.
'As discussed earlier, putty filling is generally not advisable unless the surface is poor, uneven or has pin holes. It is advisable to use primer
surfacer, which provides a thicker coat and helps to fill such surfaces. The epoxy primer is thicker and may do this job in most cases.
(No. 300 to 400) and then to wash and dry it before 3 The temperature and time of curing, as indicated in
applying the second coat. Table A13.4, are indicative and for general guidance
For protective coatings, such as with epoxy paints, only. They may vary with the type and quality of
the minimum thickness of paint (primer plus paint) paint and effectiveness of the furnace. For exact details,
should be around 70-80 microns, which is also consult the paint manufacturer. The operator may also
possible through one coat each of primer and paint. vary the given parameters slightly, based on his own
2 If different paint shades are required on outer and experience and the end results.
inner surfaces paint any one side first, cure it almost 4 Curing time will also depend upon the thickness of
completely and then apply the second shade on the coat and the shape of the workpiece.
other side. Even when there is a wrapping on the 5 Air-drying paints are easily available as noted above
second side, can be easily wiped and cleaned, without but they may not suit a regular production line as:
affecting the first. They would require a longer drying time (slow