Retarder is just very slow evaporating thinner. You should never add more than 10% retarder to your mix. That means that 10% of you total thinner aded to the paint is retarder. Adding more will allow the thinned paint to soften preceding coats and endanger plastic canopies. By adding retarder the paint drying time is slowed allowing moisture to escape before the paint skins, trapping moisture into the layer of paint. I very seldom use retarder choosing to spray only when humidity is below 50%. While some bloush might occur, succeeding coats usually make it disappear.