Do I need to thin the varnishes?
The varnishes do need to be thinned, as they are made thick to prevent the matting solids from "crashing" to the bottom of the container while it is on the shelf. How much to thin depends on what you are varnishing and your personal tastes. We suggest to first-time varnishers to thin 5 parts varnish with 1 part solvent. Thoroughly mix and test the feel of it in your brush. If you think there is too much brush "drag", or resistance, add more solvent. We think the easiest way to measure amounts is to use a see-though jar and mark off equal parts with a ruler and marker.

How do I thin down and apply varnish?
Each varnish and its application method require different approaches. Brush application: Thin with water (Polymer Varnish), or Pure Mineral Spirits (MSA Varnish) 10-20%. Satin and matte varnishes require more thinning than the gloss version does.

Spray application: Thin with 25-40% depending on the spray equipment and personal preferences.

Can I use "paint thinner" which is 100% Mineral Spirits to thin MSA Varnish?
You can use either turpentine or 100% pure mineral spirits. Avoid anything stating "low odor", "odorless", or Turpenoid. These are simply not strong enough to be compatible with the MSA Varnish. If you are concerned about a solvent being appropriate, blend 1 part varnish to 1 part solvent in a small glass container. If you see it become cloudy (note: inherent cloudiness of satin and matte varnish is O.K.) and/or separates quickly, it may cause application problems and an uneven sheen.

I use Galeria Windsor & Newton Acrylics on paper glued on wood as my painting technique. The last time I used a mixture of 50 % water/50 % Polymer Varnish with UVLS (matte) over my painting, the varnish left some whitish trails on the surface of my painting. I never had a problem before now with it. Maybe because I mixed a new bottle with an older bottle of the product (could it be possible?).
A 1:1 mix of varnish to water is very thin for brushing onto the surface of a painting. This thin of a mixture causes approximately 70% or more of the film to evaporate. This in itself can cause uneven films and gloss variance. Also, the varnish will readily absorb into porous surfaces like paper and leave behind the matting solids. This causes a frosted appearance to develop. We also think if you have made up this 1:1 mixture and it has sat around for a while, the matting solids have settled to the bottom of the bottle. If not completely remixed before using it again, you may have streaks and uneven gloss. You may have over concentrated the matting agent at the bottom as you used up the varnish, and as you have reached the bottom of the container, you then stirred up the excessive matting solids and it left a frosted quality, especially in the thicker sections.