Hello Rich,
I do not use gloss varnish, only matt. However, I know the recipees to convert gloss into matt. Also, durability of a varnish or flexibility is not so much a matter of gloss or matt but of the the type of resin the varnish is made from.
I will lecture a bit here - maybe somebody will find it helpful.
The trick to turn gloss into matt - trick employed by the manufacturers - is to add a surface structuring agent. Gloss is obtained by a liquid surface drying into a perfect smooth surface. When you add additives which modify the surface texture (basically make bumps inside) you can achieve different degrees of matt. These additives either have an effect on rheology or they are just solid elements which stick out of the dried paint surface. This latter effect can be used by anybody having access to such products (usually they are sold B-to-B and not available in your local supermarket). Silica matting agents work quite well (example Aerosil(r) pyrolized silica). There are many different. Many have the drawback that they settle in the paint bottle. So very thorough mixing or stirring before use is key. You need to determine the amount to mix into which gives the best result for your taste, and when you add them in the beginning you need to stir a long time to really incorporate them by wetting the surface - some physical work
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Concerning the durability and resistance of the varnish: Traditional glycero-phtalique paints, solvent borne, are harder, but will yellow and can crack more easily. There are new generations of such paint with less solvent and better scratch resistance, but difficult to obtain in local shops.
Water-based acrylic paints and varnishes are my personal favorites. No solvent, no yellowing and better flexibility. I use standard water based varnish for art crafts and painting - readily commercially available and cheap. Also, one layer is not necessarily very resistant, but the interaction of several layers is.
When I require very high resistance and durability I use urethane modified acrylic paints, water based. I use leftovers from wooden floor sealings. These are just fabulous. However, they are gloss or "satin" = slightly matt. Here I add matting agent to obtain my matt level. These floor sealing resins (you buy them in liters or gallons, but with 200 ml you can last for several brigades to paint) are mostly tainted in ocre, or wooden colour. My experience is that this taint is nearly not visible because of the thin surface. Also, these varnishes come with thickeners - the paint without dilution can be applied more thickly. Usually, I don't want this and it is easy to water down the paint to a desired viscosity level.
Greetings
Michael
If I want to obtain a very robust varnish