There is a tool called a "wet mil gauge."   It's usually a credit-card sized piece of metal with notches all around.  The purpose is to measure the thickness of the finish, when wet, in "mils" (0.001")   You simply hold it down on the wet finish and see where the prongs no longer leave an indentation.

 

Why is this important?   Too thick a finish will run or curtain and may not cure properly. Too thick a finish may also be prone to cracking.  It is probably most important when spraying on a finish because you don't have that tactile feedback that you get with cloth or brush.  If you can get the technical data sheet on a finish, it will often tell you recommended wet film thickness (and resulting dry film thickness).

 

You can get these for a few dollars from a good paint store or supplier, sometimes even free.

 

 

How to use: https://www.shellac.net/wet_mill_gauge.html

 

 

 

I didn't know that, thanks.

Herb

I never even thought of this, I have experience in the past those issues tool, specially curtains. This is a wonderful handy lil tool, thanks Keith.

I give up. What's a curtain? I'm guessing a sag? 

57 minutes ago, Gene Howe said:

I give up. What's a curtain? I'm guessing a sag? 

Yep

Like I said, I am learning so much from these posts- Thanks!