Dry Film Thickness

What are you using to measure dry film thickness? Are you happy with it? What are the pros and cons?

Outside of a finish manufacturer or consultant I doubt you will find many takers for that question. My best guess for dry film thickness would be based on a wet mil gauge x % solids x # of coats. You might try posting on the Woodweb finishing forum. What are you doing that requires such a measurement?

Kevin ... just looking for a reasonably priced coating thickness gauge (paint meter). One of the popular ones is Elcometer, but there are lots of others including some very low priced units. Typically they are called a "DFT" (dry film thickness) gauge. Most DFTs for wood or plastic use ultrasonics to measure the thickness. Knowing the thickness is important for certain types of sprayed on finishes because of their sensitivity to failure if applied too thick. For instance, most woodworking conversion varnishes - either solvent based or waterborne - specifically state that their maximum thickness be somewhere between 4 and 6 mil. Anything greater than that will result in failure (see warning in application instructions snippet provided below). There are lots of posts here in SMC about these failures with CV.

This came up when looking at a post for protecting a photo-reactive wood from sunlight by using a UV inhibiting finish. The finishes discussed that had UV blockers both happened to be waterborne conversion varnishes. Both had (and of course now I can't find the cautions) instructions not to apply over 4 mils dry. So, how does the small-time operator that has found these finishes to work really well ... verify they have applied the finish correctly? While I haven't had any finish failures I'd love to have the confidence that I've followed all the instructions provided.

I should probably add that when possible, I take my work to a professional to apply the finish.

Bill,

In my experience professional finishers control their mil thickness by eyeballing the wet coat and keeping the number of coats to a minimum when using CV. Some have a wet mil gauge buried in their toolbox, but I have never seen a non-destructive dry mil tester in a shop. 4-5 mils wet is as thick as most can be sprayed without running on a vertical surface and 3 coats is standard. 4 coats at most with sanding between seems to work in the real world. Typical solids content is 25-30% before thinning, so you can do the math to estimate final dry thickness.

I don't mean to poo poo the idea of having a tester in the shop, especially if they are affordable. My point is simply that you are likely posing the question to the wrong audience. I would suggest asking the techs at various finish manufacturers what they use.

Kevin ... that reflects my approach and simple logic. Likely it is working since I haven't had a finish failure. I'll ask my finish supplier, but your clear explanation makes it easier to do that. Thanks!

I've never done it but see no reason you couldn't use a micrometer to measure the dry thickness as long as you measure the substrate prior to finishing. I would do this on test specimens sprayed at the same time as the project, and I would put circles or some other reference mark on the specimen so that I measured each coat in the exact same spots to eliminate the impact of substrate thickness changing from spot to spot.

John

Bill, a dft gauge is an essential part of my kit as is a comb type wet film gauge. Same goes for the boys in my shop. Get one that has the correct probe for the class of substrate. You are dead right to be concerned about over application with certain products causing failures. Following manufacturers instructions on data sheets is actually not a bad idea.

I have a Positector 6000 for serious, verifiable testing and a no-name one for routine checking between coats. I have used Elcometer gauges in the past. They are just as good. See what deal you can get.

The wet film gauge is the best tool for controlling thickness during application. Everyone thinks they are perfectly even sprayers until they use a wet gauge. As for no-one doing any testing, that gets you sacked from my shop. Cheers

I posted a query at Woodweb on this subject and got several interesting responses http://www.woodweb.com/cgi-bin/forum...pl?read=813943.

Thanks Kevin, a good read. My takeaway is that the issue is indeed real and should be a concern, and that using some sort of test (wet or dry) is a good best practice for providing feedback on one's finishes.

I feel using the wet mill gauge is best and get used to the way it looks at the right thickness I have never had a problem with my finish thickens and its how nearly all finish companys will tell you to do this, the wet mill gauge is under 10 dollars. the reason I like this way also is when spraying is when you want to know what you have so you can adjust as opposed to fixing things later.