We do not recommend using universal thinners. The use of a thinner with an unknown composition may result in loss of adhesion, lack of proper flow of paint and varnish defects.
Why did the polyurethane topcoat become dull?
Possible causes
Too short time for evaporation of the thinner,
How to prevent
Maintain the right conditions for complete evaporation of the thinner,
How to remove
Mat the surface, degrease and apply the paint according to the rule (right conditions and thinning proportions)