I've used POR15 on other rusted things - above ground fuel tank, mower deck (top), and by following their directions, it's worked very well. My concern in the culvert is how do you get it 100% dry.....

http://www.por15.com/POR-15-Rust-Preventive-Paint/products/1/

www.hoosierpondpros.com


http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).