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Computing Film Thickness and Coverage
o, you want to know how much
paint to apply to a surface, but to
find out, you must deal with a variety of
numbers and calculations concerning
such things as wet film thickness, dry
film thickness, solids by volume, and
theoretical coverage.
How did this all get so complicated?
Actually, it’s not all that bad. This month’s Applicator
Training Bulletin explains how to calculate film thickness
and coverage.
✔
Solids by Volume
All coatings have solids, which form the coating film
when it dries or cures. Solids content can be given by
weight or by volume. The manufacturer needs to know
the solids by weight to determine if the paint has been
made properly. The painter needs to know the solids by
volume to calculate dry film thickness and coverage area.
Solids by volume is normally given as a percentage of
the total volume of the paint. For example, assume you
have 1 litre of paint, and its solids by volume is 75%.
That means if you were to extract everything except the
solids content, you would have only 750 mL of material
left in the container.
It also means that if you measure the wet film thickness
(WFT) at 100 microns, the paint film will be only 75 microns when it dries.
There is a simple way to calculate how thick you need
to apply paint to get the desired dry film thickness (DFT).
The formula is
DFT = WFT x solids by volume
This is the equation used in the example above. We
multiplied the WFT (100 microns) by the solids by volume (75% or 0.75) to get the DFT (75 microns).
Of course, if you have to add thinner to the paint, the
solids by volume will be less. The formula to determine
the solids content of paint after thinning is more complicated than the equation above, so a table in which the
calculations have already been worked out often is used.
For example, assume you want to thin a coating with
75% volume solids by 10% and then determine its new
solids content. Using Table 1, find the row for 75% volume solids along the left side and read across the row to
the column for 10% thinner added. The row and the column meet at 68%, which is the solids by volume of the
thinned coating.
Calculating DFT
Now that the paint is thinned, and you know that the
new solids content is 68%, you can determine how thick
60
This month’s column, which originally appeared in
the July 1991 issue of JPCL, has been revised and
updated by PCE European Editor Brian Goldie.
the wet coating film must be to achieve the specified dry
film thickness. (Coating thickness specifications are written in DFT.) Table 2 presents this information based on
the formula already mentioned (DFT = WFT x solids by
volume).
When you look at Table 2, you will see that there is no
68% along the far left column, which is the solids content
of the thinned coating. So you should round to the nearest
number, which would be 70%. Assume that the coating
specification being used requires a DFT of between 150
and 250 microns. (Note: these numbers are selected for
purposes of the example; most coating specifications do
not allow this much of a range in coating thickness.) You
can determine the required wet film thickness by reading
across the row marked 70% to the columns labelled 150
and 250 microns. By doing this, you can see that the wet
film thickness must be between 214 and 357 microns. A
good painter should be able to stay within this spread.
Calculating Coverage
Now, knowing the solids by volume of the paint and the
coating thickness range, you can estimate how much paint
you will need by using Table 3.
Once again, assume the volume solids of the thinned
Table 1:
Solids Content After Thinning
Thinner added
Original solids content of material before adding thinner
S
35%
2%
5%
7%
10%
12%
15%
17%
20%
100%
98
95
93
91
89
87
85
83
80
77
74
95%
93
90
89
86
85
83
81
79
76
73
70
90%
88
86
84
82
80
78
77
75
72
69
67
85%
83
81
79
77
76
74
73
71
68
65
63
80%
78
76
75
73
71
70
68
67
64
62
59
75%
74
71
70
68
67
65
64
63
60
58
56
70%
69
67
65
64
63
61
60
59
56
54
52
65%
64
62
61
59
58
57
56
54
52
50
48
60%
59
57
56
55
54
52
51
50
48
46
44
55%
54
52
51
50
49
48
47
46
44
42
41
50%
49
48
47
46
45
44
43
42
40
39
37
45%
44
43
42
41
40
39
38
37
36
35
33
40%
39
38
37
36
36
35
34
33
32
31
30
35%
34
33
32
31
31
30
30
29
28
27
26
30%
29
29
28
27
27
26
26
25
24
23
22
25%
24
24
23
23
22
22
21
21
20
19
19
PCE July 2000
25% 30%
Copyright ©2000, Technology Publishing Company
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Table 2:
Wet Film Thickness Requirements
spray systems, the loss can
be even higher.
150
375
50
75
100
125
175
200
225
250
275
300
325
350
400
425
If you are painting struc150
375
75
100
125
175
200
225
250
275
300
325
350
400
425
100% 50
158
395
53
80
105
132
184
211
237
263
290
342
352
368
421
447
tural
steel, you can expect
95%
167
417
83
111
139
194
222
250
278
306
361
361
389
444
472
90% 56
more loss from overspray
177
441
88
118
147
206
235
265
294
324
382
382
412
471
500
85% 59
than when you are painting
188
469
94
125
156
219
250
281
313
344
406
406
438
500
531
80% 63
large, flat steel plates. Gener200
500
100 133
167
233
267
300
333
367
433
433
467
533
567
75% 67
ally, the smaller the work214
536
71
107
143
179
250
286
321
357
393
464
464
500
571
607
70%
231
577
115 154
192
269
308
346
385
423
462
500
539
615
654
piece is, the more loss you
65% 77
250
125 167
208
292
333
375
417
458
60% 83
will have from overspray.
273
136 182
227
318
364
409
455
55% 91
Also, if you are applying
300
250
350
400
450
50% 100 150 200
primer
to blast-cleaned steel,
333
278
389
444
45% 111 167 222
you may need as much as
375
313
438
40% 125 188 250
20–30% extra paint because
429
357
35% 143 214 286
417
30% 167 250 333
of the extra surface area
25% 200 300 400
from the anchor pattern.
Note:
film film
Thicknesses
are minimum.
No allowance No
is made
for evaporation
of solvents
during application.
Note:DryDry
thicknesses
are minimum.
allowance
is made
for evaporation
of solvents during application.
These are rough estimates.
Experience is the best guide
paint is 68% (rounded up to 70%) and that the specified
for estimating the loss factor to be expected on a particuDFT is 150–250 microns. Using 200 microns as an average,
lar job.
follow the 70% row in Table 3 across to the 200 microns
DFT column. You’ll find that 1 litre of paint should cover
3.5 m2 of surface area. However, as the table heading
notes, this is the theoretical coverage area. This means 1
litre of paint will cover 3.5 m2 only if you use all the paint
in the pail, leave none in the hose, lose none to overspray,
don’t spill any, and apply it to a very smooth surface. What
are the chances of all that happening?
To get a realistic coverage rate, you must include a “loss
factor” in the calculations. Depending on conditions, the
loss factor can be as little as 5% or as much as 50% or
more. Normal loss is 15–20%. For this example, let’s use
20%. To calculate realistic coverage, multiply 3.5 m2 by
20% (0.20). Then subtract the result (0.7 m2) from 3.5 m2.
This gives you a realistic coverage rate of 2.8 m2 per litre
of paint.
How many litres of this paint would you need to cover a
particular surface? Measure the surface area and then divide it by the realistic coverage rate. For example, if the surface area to be painted is 290 m2 and the coverage rate is
2.8 m2 per litre, you would need 103.5 litres of paint, which
would be rounded up to 104 litres, of course.
This example uses a 20% paint loss factor to calculate realistic coverage. The loss factor is very important in estimating the amount of paint needed for a job, and many
things can affect the loss factor. For example, applying the
paint by brush or roller means very little paint loss, mainly the paint left in the can. Airless spray application, however, can result in a loss factor of 15–20% from the paint
that rebounds from the surface, and with conventional
Solids content after thinning
Dry film thickness (microns)
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PCE July 2000
61
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Table 3:
Theoretical Coverage in Square Metres per Litre
Solids content by volume
Required dry film thickness per coat (microns)
25
50
75
100
125
150
175
200
225
250
275
300
325
350
375
400
100%
40
20
13.3
10
8
6.7
5.7
5
4.4
4
3.6
3.3
3.1
2.9
2.7
2.5
95%
38
19
12.6
9.5
7.6
6.4
5.4
4.8
4.2
3.8
3.4
3.1
3
2.8
2.6
2.4
90%
36
18
12
9
7.2
6.1
5.1
4.5
4
3.6
3.2
3
2.8
2.6
2.4
2.3
85%
34
17
11.3
8.5
6.8
5.7
4.9
4.3
3.7
3.4
3.1
2.8
2.6
2.5
2.3
2.1
80%
32
16
10.5
8
6.4
5.4
4.6
4
3.5
3.2
2.9
2.6
2.5
2.3
2.2
2
75%
30
15
10
7.5
6
5
4.3
3.8
3.3
3
2.7
2.5
2.3
2.2
2
1.9
70%
28
14
9.3
7
5.6
4.7
4
3.5
3.1
2.8
2.5
2.3
2.2
2
1.9
1.8
65%
26
13
8.7
6.5
5.2
4.4
3.7
3.3
2.9
2.6
2.3
2.2
2
1.9
1.8
1.6
60%
24
12
8
6
4.8
4
3.4
3
2.6
2.4
2.2
2
1.9
1.7
1.6
1.5
55%
22
11
7.3
5.5
4.4
3.7
3.1
2.8
2.4
2.2
2
1.8
1.7
1.6
1.5
1.4
50%
20
10
6.7
5
4
3.4
2.9
2.5
2.2
2
1.8
1.7
1.6
1.5
1.4
1.3
45%
18
9
6
4.5
3.6
3
2.6
2.3
2
1.8
1.6
1.5
1.4
1.3
1.2
1.1
40%
16
8
5.3
4
3.2
2.7
2.3
2
1.8
1.6
1.4
1.3
1.2
1.2
1.08
1
35%
14
7
4.7
3.5
2.8
2.3
2
1.8
1.5
1.4
1.3
1.2
1.1
1
0.95
0.9
30%
12
6
4
3
2.4
2
1.7
1.5
1.3
1.2
1.1
1
0.9
0.9
0.81
0.8
25%
10
5
3.3
2.5
2
1.7
1.4
1.3
1.1
1
0.9
0.8
0.78
0.7
0.68
0.6
Conclusion
If you know the solids content of the paint you are using
and the specified DFT range of the applied coating, you can
62
use the information presented
in this article to calculate the
following:
• solids content of the paint
after thinning;
• WFT range needed; and
• theoretical coverage area.
By estimating the anticipated
loss factor, you also can calculate the practical coverage
area, and then, by measuring
the size of the area to be painted, you can calculate how
many litres of paint you need.
However, how close these calculations are to reality can be
affected, of course, by the experience and competence of
the painter on the job.
Next month: What you should know about
applying water-borne coatings
PCE July 2000
Copyright ©2000, Technology Publishing Company