Just because it's so much fun to pick nits...
In the auto paint business, I found that most people in the business would refer to reducer if it was used to thin the mix of either enamal, acrylic enamel, or urethanes.
When you were talking about lacquer paint, then you'd refer to it as thinner.
I think that came from the fact that reducers and thinners really were different products, chemically. Using a reducer in lacquer paint would end in failure, although you could thin acrylic enamel with lacquer thinner and it would dry quicker. Of course, the finish would fail within months, sometimes weeks (no, I never did that myself; I just saw the results once in a while).
The reducer used in urethane paint is NOT a catalyst; that's a different additive. The purpose of the reducer is to achieve the correct viscosity for spraying. Also note that reducers come in different blends tailored for different ambient temperatures.
Using a "cold" reducer in hot weather will result in poor flow and solvent trapping, and using a "hot" reducer in cold weather will slow the flow so much that you would be much more likely to encourage sag and runs (not to be confused with sag and runs with reference to some of us older folks
).
Retarder is usually only used in a last mist coat to lay down overspray when blending onto an existing finish but if you haven't painted much, you might do more damage than good if you over do it.
Some people think that retarder is an additive for the primary paint mix but if you do that, you slow down the flash time so much that you're begging for sags and runs first, then more dust in the finish because of the too-long open time of each coat, then a real possibility of solvent trapping which means problems that would require stripping and starting all over again.
I love answering these questions because they continue to remind me why I'm so glad I got out of the auto paint business over 10 years ago after busting my butt with nothing to show for it after more than 25 years.
Thanks! I needed that.
