Recommendations for a frame rust protecting coating?

[UKlandy]

I want to rust protect my truck but I am not really sure what to use during the processes. Does anyone have any good suggestions of products they have used before?

 

[Rockingladiator]

When I did mine I just cleaned the whole underside and sprayed over with a 3M Undercoat. It worked great and is still on there after 2 years