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Does adding reducer to a top coat increase the sheen or does it reduce it ?
It shouldn't do either. But in reality because it slows down the drying it gives time for the flattening paste to sink to the bottom of the film and increase the sheen a bit. But that only happens in extreme cases when you have a lot of reducer in the mix.
Adding reducer to your topcoat doesn't change the way that flatting paste works in your film-forming finish. Adding retarder and/or flash-off control agents can raise your gloss level (give you more of a shine). Adding fisheye remover can also increase your shine.
Flatting paste works at the surface level of a cured film by diffusing the light that bounces off of the surface. When the flatting paste sinks into your flashing finish because the finish is open longer than the formulators intended, less light will be diffused, hence more distinct reflection on the surface of the film, and therefore more shine.
At the same time, the flatting paste is still within the coatings. If too much is there then your coating will start to gain a milky appearance.
What all this means is that you can take a dead flat finish, one that has been loaded with flatting paste, and polish it to a brilliant shine because you're "smoothing" a surface that was once "micro rough," this so-called roughness created by the presence of the flatting paste at the surface of the flashed-off film. It's all about texture, even on a microscopic level.
That said, you can't polish out a cloudy film, one which is loaded with flatting paste.
It's this explanation that's behind the school of thought that one should build his finish with gloss and only the very topcoat of finish should be be in the sheen that you want.
The amount of milkiness and potential cloud within a finish that's only slated to be 4 - 5 dry mils thick is inconsequential in an overwhelming majority of circumstances.
I once spot sprayed my semi-gloss lacquer on a table top, misting it on in light coats, prior to a full topcoat of the same sheen. My spot-sprayed area was cloudy, it was ugly, it was visible. The milky look didn't go away after topcoating. Lesson learned the hard way.
Thanks for the responses guys .