There are several products you can use under your fender. One that is preferred is Linex bed liner. It protects the under side from rust, but it also prevents "stars" in the paint from forming on the top side of the fender. Stars are the result of rocks being thrown against the fender.

Another product is an under coating By 3M. It comes in an aerosol can. One can will provide about 3 coats. After it cures, you can either leave it black or paint it.

Another idea that most people use is just paint it with tractor paint that you get at Tractor Supply. It's tough paint and safer than POR-15. And a lot less expensive.

Frank Cox
Cox Custom Cars

No better smell than that of fresh paint. I've been building and painting cars/trucks for 47 years. Not gonna stop now.
http://www.coxcustomcars.com/