[B Coll]

As for setting up your spray gun the first thing I would recommend, especially if you lack experience, is to get a viscosity cup. Literature should be available as a guide to recommend what viscosity your unit can handle and the air setting for that as well as needle/nozzle/cap sets recommended for the given viscosity. I would suggest checking the viscosity before thinning. If that is within your cap set fine. If the viscosity is high you can increase cap sets or add reducer realizing that a smaller cap set generally creates finer atomization. Do not thin beyond the manufacturers specs to achieve your target viscosity. You may also need to add a retarder to decelerate drying and that will also reduce viscosity. If your plan is to spray a solvent based product like oil based poly, I would much rather spray pre-cat lacquer. Make sure you read about safety when spraying solvent based material. An organic vapor mask with a paint filter is a must and a Tyvek suit would be recommended. Excellent ventilation is a must and in the case of lacquers all precautions need to be taken to eliminate any source of spark or flame and have excellent ventilation to avoid the risk of explosion. I agree with GC Tony that in my opinion oil based poly is on the bottom of the list of finishes I would spray. I would recommend looking at Target Coatings or General Finishes.