Do I need to thin the material?

Transparent and semi-transparent stains should not need to be thinned. If thinning is necessary, as little as 5% can make a big difference. Use water for water-based materials and mineral spirits for oil-based materials. Never exceed the thinning advice given by the coating manufacturer. You can also add a flow agent, like Paint Easy by Wagner (this is not a thinner) for water-based material. Make sure your material has been properly mixed and strained.