Questions: 1. Which is better and why? 2. Will it plug up pin holes, etc.? 3. Can I or should I spray the interior of the lowers and uppers - will it affect tolerances? Use: AR-15 new build - both upper and lower. Reason: desire a more scratch resistant and durable finish than factory. Color: traditional AR black. Tools: I have an airbrush and compressor. Thanks so much for your time, wisdom and knowledge!