Measuring and Calculating Mil Thickness of Finish Coats

A little explanation of how to gauge wet or dry mil thickness. December 31, 2013

Question
Could someone give me a little education on mill thickness? I spray a lot of lacquer and have sprayed some CV but am no professional finisher. When you say CV should be a max of 3 mils, how do you gauge that? Dry mils, wet mils, etc.?

Forum Responses
(Finishing Forum)
From contributor J:
You need to do some math with a wet mil gauge, or get a device called a positector to measure the dry film. I will assume you want to measure the wet film as it costs almost nothing. Most vendors will give you a wet film gauge for free. When you spray a part, you touch the wet film gauge to the part while itís wet (as soon as possible) and take a reading. You will probably find you are laying down a wet coat of about 3 mil. Now, find the solids by volume content of the product you are spraying (ignore solids by weight, itís a marketing measurement - you need solids by volume).

Letís assume your product is 30% solids by volume. Now for the math - wet sealer coat at 3mil and when dry, 30% of 3mil is 0.9mil so your first sealer coat has a dry film of 0.9mil. Scuff sand and remove a bit of that dry film. Letís say you are now at about 0.6mil. Spray another 3mil wet coat, and when dry you will have a total film of 0.9mil plus 0.6mil from the seal coat for a total dry film of 1.5mil. Keep subtracting a bit for scuff sanding and adding the 0.9 dry mil for each 3 wet mil coat sprayed and you will arrive at a very close approximation of your total dry film thickness.

Now if you add thinner to your product before spraying, you are reducing the solid content by volume of the product. Say you add 10% thinner to the mix. Your 30% solids by volume will now be reduced by 3% for a total of 27% solids. Typically for most lacquers and conversion varnishes, the product data sheets will recommend applying wet coats in the area of 3 to 5mil and they will recommend keeping total dry film at less than 5mil. Refer to the data sheets for the particular product you are using and this information should be there. You want to look for wet coat, total dry film, and solids by volume percentage. Combined with the math above, the only thing you should need is a wet film gauge. Just ask your finish supplier for one. It will either be free or about $5. If you exceed the film thickness recommendations, you can run into all sorts of problems such as cracking, lifting, wrinkling, etc. and some of the issues may not show up for a month or so after spraying.



From contributor F:
I would think you would have to scuff sand pretty hard to remove a significant amount of mil thickness in the overall coat. I never account for scuffing as it's impossible to judge how much you are removing. I see a lot of finishers just barley scuff with a sponge pad these days.


From contributor J:

I agree. Though it will remove some you just canít be certain how much without a positector. I would think 0.3mil would be a reasonable guess though, even if only using a quick swipe with a sponge. Itís really irrelevant anyways. Even if you were to apply 5 wet coats at 3mil, you would still be under the 5 mil dry as recommended by most. Especially since most of us are only applying 2 or 3 coats on most jobs. These thickness limits really only become a concern if you have some crazy finish schedule that would require you to apply really thick coats, or lots and lots of coats, or you canít apply an even coat due to operator error (get to heave along edges, or center due to poor technique).