I find that almost all clears are just "cheap".Spend hours getting a paint job looking nice and 12 months later the Aussie sun has made it look like it has been sunburnt and it's peeling........Sheeeeeit!!!!!
My paint supplier buys it generic 20 liter drums and if i don't insist on a decent product,i get the "car yard " special in a house of kolor tin.
I have had a flat black car (paint product unknown ) sitting in my yard for 18 months in all sorts of weather and NO rust has appeared ,So i don't think all matt or satin finish auto paints are porous,just acrylic primer.