Use rivets of the same type metal as you are riveting to avoid dis-similar metal electrolysis problems. There are flat head rivets (flush rivets) that fit into a specially prepared depression made in the two pieces of adjoining metal and must be bucked on the back side. They are quite a bit more trouble to use. In general, there is no good reason to use flush rivets on a trailer unless one wanted paint the finished product and have no rivets showing. Because bucked rivets require access to the back side of the panel, repairs are often done from the outside with a special type of rivet that uses a breakaway mandrel-a type of "pop" rivet. These are made by an outfit called Olympic and use a special shaver(optional) to profile the head after the rivet is installed.
"Not all who are laundering are washed" say Bill & Heidi
'78 Excella 500,"The Silver Pullit". vacuum over hydraulic disc brakes, center bath, rear twin. '67 Travelall 1200 B 4X4 WBCCI 3737