No, you do not need to thin Valspar One-Coat Solid Stain & Sealer when using the Wagner Flexio 2500 sprayer. This stain is formulated for application with professional spray equipment, and thinning is not recommended. Instead, adjust the sprayer's settings—such as air pressure and material flow—to achieve optimal coverage and finish. Always perform a test spray on a small area to ensure the desired results. Thanks for your question.