I treat my boards with liquid tin before I solder them. Polish the copper with a scotch brite pad and let it set in the solution for about a minute. Then clean the board in water, dry and give it a thin coat of Kyrlon clear lacquer. This keeps it from oxidizing and is very easy to solder through. For surface mount devices the Krylon also works somewhat like a solder mask. I get the liquid tin from Amazon. https://www.amazon.com/MG-Chemicals-421-Liquid-Bottle/dp/B005T8PE4A/ref=sr_1_1?ie=UTF8&qid=1469047958&sr=8-1&keywords=liquid+tin DuWayne kv4qb.blogspot.com