Rust free coating for gutter

[Vimal]

Hi,

I am going for a GI gutter for my home slope roofs to harvest rain water. I need to go for a special coating to the inner surface of the gutter so that it will be rust free. The gutter fabricator suggests either Duco paint coating or FRP sheet lining. However, both the options are costly. I would like to find out if there is an economical option available. Any suggestions ?

Thanks
Vimal.