Wet Mils, Dry Mils, and Quality Control

Finish coat thickness is a critical issue for coating performance. Here's some info on how to measure thickness of coats, wet and dry. July 17, 2008

We use a Del Fesko PosiTector 200 device to measure coating thicknesses. The typical guideline for CV (or lacquer, for that matter) is between 4-5 wet mils or 1 dry mil. What does this refer to, each coat of sealer and/or finishing material? If not, does that mean you spray 2 mils of each?

Yes, I have tried to ask the rep - in fact, several reps - of the top companies at the shows. They have had no clue.

We currently spray with an HVLP/pot setup and with an HVLP gravity gun. Either way, after the wiped-on stain, the process is to spray the edges of doors or panels first and then one pass lengthwise and one pass across on each side at a moderate speed for the sealer coat, and we repeat for the finishing coat. We use Valspar CV, which is its own sealer. We end up with about 2 mils when dry. It looks and feels great. 1 mil would look miserable. We have had no problems with the finish. Are we technically overdoing it?

Forum Responses
(Finishing Forum)
From contributor P:
Two mils dry film thickness (DFT) is fine. The tech data sheet probably has a warning not to exceed 4 or 5 mils DFT. If you do, the finish is likely to crack.

You can measure how thick you spray each coat using a wet mil gauge from your coatings supplier. If it's not free, it should only cost a few dollars. After using the gauge for a few times at most, you'll have a good feel for how thick you're spraying by the way it looks. You can check yourself every once in a while with the gauge just to confirm nothing has changed.

You can calculate the DFT by using the solids content (by volume) of the finish and how thick you spray each coat (wet mils). For example, if the finish has 30% solids by volume and you spray a 5 mil coat, then only 30% of those 5 mils will be left once the solvents evaporate (the other 70% of the finish). Multiply 30% (0.3) by 5 and you get 1.5. So spraying a five mil wet coat leaves you with a 1.5 mil dry film.

If you scuff sand and spray another 5 mil wet coat, you'll have a total DFT of 3 mils. It will actually be slightly less as a result of the sanding between the first and second coats.
If you use a sealer under the CV, you'll have to calculate its DFT separately since it most likely has a lower solids content. Calculate the DFT for the sealer and the CV topcoat(s) and then add it all together to make sure you stay under the 4-5 mils limit stated in the data sheet.

From the original questioner:
Thanks. The thing is, the spec sheet specifies one dry mil period. I am getting the Kremlin demoed tomorrow, so it would be a good time to test the wet mils vs. the dry mils.

From contributor J:
I recently attended and MLC spray school. Here is what I was told. You want a finish between 3.5 and 5 mil when dry. Spray around 5 and 6 mil wet. For their Krystal conversion varnish, which is about 40% solids, 1 wet coat at 6 mil will be ~2.5mil when dry. Therefore, first seal coat is ~2.5 mil dry, scuff sand which removes a little of that, then a second coat brings you to just under 5 mil dry. Using something with lower solids may require 3 or more coats to get the 3.5 to 5 dry mils. I should hope your finish that says 1 dry mil means 1 dry mil per coat, but that even sounds too low. What is the solids by volume of the finish? What is the finish?