Screen out of mod wood

Hi i need to build a screen across my back yard wich is approx 20 mtrs wide

I want to screen a large 25000 lt water tank, clothes line and shed

i have installed some 75 x 75 x 2.5 gla post at 1200 mm apart

I am thinking of laying some 68 x 17 mm modwood slats on it

I intend to plant some shrubs in front of the screen to soften it down a bit, so this will make it hard to keep up its apperance with oil etc on it, so i think using timber is not a good idea.

So i was thinking of using modwood for the slats in a redish colour.

Has anybody had any experiance with this material, does it sag and what type of screws do you use to fix it to the gal post

any info would be appreciated