POR-15 is a great product. It does as it says it will do. Stops rust. At least in our experience it works very well in our application on vehicle frames that still have some amount of pitted rust even after the whole thing had been thoroughly cleaned of grease and sandblasted to remove large particles. The ONE thing that we would NEVER tell ANYONE to subject themselves to do with this product is to spray it. EVER. Even in a well ventilated perfectly designed booth. Even outdoors. The volatility of this compound sticks with your lungs for days on end. Their application instructions tell you to spray it and it is faster to do so when covering large areas, but we will never do it after a single first hand experience that was not a good one.
So how do you apply POR15? We found that foam brushes work very well for application to the main surface areas and since these brushes come in varying sizes they also allow you to get into the smaller harder to reach locations as well as cover large surfaces efficiently. They are also cheap, so cleaning is not required. Just throw them out when you're done. We had the whole vehicle dismantled, so there weren't parts to get around with the foam brushes, but you want to tape off surfaces you're not intending to cover with this product.
Good luck. Save your lungs. Brush it on.
For the manufacturers info see this link to FAQ's: http://www.advanced-rust-protection.com/faqs.html#applic