TotalBoat Gleam spar varnish should be thinned ONLY with TotalBoat Special Brushing Thinner 100 for brushing applications. Similar products on the market include Interlux Brushing Liquid 333, Pettit Brushing Thinner 120, and Epifanes Thinner for Paint and Varnish (for 1-part products only), but TotalBoat can not guarantee the same product satisfaction or application qualities if any of these products are used to thin Gleam.
Do not use mineral spirits, acetone, denatured alcohol, toluene, MEK, or generic hardware store paint thinner. Xylene can be used for spray applications, but it will not give ideal flow or working time for brushing applications.