I just did this to mine. You need to take a look at the box of staples. The stainless staples I found said right on them NOT for marine use. So I researched a littlefurther and found the best staple to use is a Monel.
http://www.bing.com/shopping/arrow-t...es&FORM=HURE.I used the 3/8. Worked out great. A electric staple gun will really help. I used close to a 1000 staples for 6 boards total of 34ft of board. I put the staples close and heavy on the ends to match how they did it when the trailer was made. I could never imagine doingitwith a manual staple gun.