Go to Tractor Supply and get some
Rust CutterSpray or brush/roll it on the chassis (make SURE you wear a respirator mask!), let dry for a few hours, and then paint over it with a Rustoleum type paint.
I did the above to my trailer chassis about 3 or 4 years ago and it still looks great today. It was quite rusted before that.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine