I am using Penofin Verde as a penetrating oil sealer for outdoor redwood furniture in California. It is a proprietary formulation, green and non-toxic. Their application instructions are explicit (http://www.penofin.com/pdf/penofin-verde-application-instructions.pdf): DO NOT add thinning or blending agents. For the best results, if time or expense is not an issue, it is standard practice with penetrating oil sealers to thin the first few applications, for better penetration. I have direct experience with Sutherland Welles exterior tung oil sealer, not a green product, but the definitive high-end oil sealer. It is much thinner than Penofin Verde, and penetrates deeper into redwood. I view thinning an oil sealer as a standard technique, and none of Penofin's business to tell me not to proceed in this way. However I would like to maintain a green approach. Some people make their own formulations using e.g. a citrus solvent. Does anyone have an educated guess what drying agent is already present in Penofin Verde? Would a citrus solvent work for me? I tried various angles, asking this on their phone line 1-800-Penofin. What do I do with a half can that has lost some drying agent to evaporation? Mix with a fresh can. A wonderful product, but they're not budging an inch on this question.