DRY FILM THICKNESS

WHAT DO THE NUMBERS REALLY MEAN

There are usually two numbers on manufacturers Product Information Sheet that most often referred to. They are the % Solids by Weight, and the % SOLIDS by VOLUME. The % Solids by Weight is a number that really is only used when mixing the formula; however it is usually higher than the Solids by Volume number so a lot of sales types will quote it to make their product more impressive…

 The number that really has any meaning to you, the consumer, is the % SOLIDS by VOLUME. This is the number that can tell us how much finish is left on the wood after the solvent evaporates. This is called the Dry Mil Thickness. It is recommended that when you add up the thicknesses of all of the coats of finish on a piece that you achieve a total Dry Mil Thickness of 3-4 Mils. It is recommended that you do not exceed 5 Dry Mils. You will notice that when you calculate the Dry Mils for Pigmented Systems that the System Total is often greater than 5 Mils. This is because the primer in these systems will be sanded back rather aggressively, so the net thickness will be less than 5 Dry Mils.    

Greater than 5 Dry Mills and a finish can become brittle, wrinkle, spider web and cause all kinds of other problems.

 It is usually recommended that when applying your finish, you apply 3-5 Mils of Wet finish onto the wood. You should use your wet mil gauge to check on how much you are putting down. Based on a 5 Mil wet coat of finish there are a number of charts that I have included to help you calculate your Dry film Thickness, number of coats needed, and also your material cost per square foot.

In all cases a coat of finish is calculated as being sprayed on 5 mils wet; with a spray gun that has 75% transfer efficiency. This is between HVLP and Air Assisted Airless. 

The numbers here are for comparison purposes. They will vary according the transfer efficiency of your spray gun, and the actual wet mill thickness that you apply.

DRY FILM THICKNESS CONVERSION CHART

If you know the % Solids by Volume, you can determine the Dry Film Thickness for a coat of finish.

 TOTAL SYSTEM FILM THICKNESS SHOULD BE 3-5 DRY MILS.  

% SOLIDS BY VOLUME

DRY FILM PER COAT

 

% SOLIDS BY VOLUME

DRY FILM PER COAT

10

0.5

 

31

1.55

11

0.55

 

32

1.6

12

0.6

 

33

1.65

13

0.65

 

34

1.7

14

0.7

 

35

1.75

15

0.75

 

36

1.8

16

0.8

 

37

1.85

17

0.85

 

38

1.9

18

0.9

 

39

1.95

19

0.95

 

40

2

20

1

 

41

2.05

21

1.05

 

42

2.1

22

1.1

 

43

2.15

23

1.15

 

44

2.2

24

1.2

 

45

2.25

25

1.25

 

46

2.3

26

1.3

 

47

2.35

27

1.35

 

48

2.4

28

1.4

 

49

2.45

29

1.45

 

50

2.5

30

1.5

 

51

2.55

Using the % solids by volume of the finish, we can calculate the dry film thickness you get from a 5 mil wet coat. This is based on the fact that at 100% transfer efficiency, 1 gallon of 100% solids finish material will cover 1604 sq ft at 1mil thick. This formula helps us determine how many coats we will need to get a total finish film that is the recommended 3-5 dry mils thick. Calculations are made using a spray gun that has 75% transfer efficiency; this is between HVLP and Air Assisted Airless.