Even though they're meant for sound-deadening, it seems like they've helped in my case. But I also spray behind them during the Winter months. I figure they help block the rocks that breach the paint.
Anyway regarding your question, have to use a layered approach. In my experiences with my prior vehicle the undercoat is good until there's a breach. The undercoat then holds the moisture beneath. Address surface rust. Etching primer, primer. Thick layers. The 3M undercoat may be best. Then use the wheel well liners but make sure they're properly secured.
https://www.feoa.net/threads/upper-strut-mounts-lasting-12-months.117467/post-1123300
Much of the ass-end of my prior car was rusted out. But no one could tell just looking at it because the OEM undercoat was still intact. I could tell by knuckle-knocking around back there with my fist. Oxidation processes accelerate when the materials are exposed to higher concentrations of oxygen. (such as liquid H²O) Heat also accelerates the oxidation process.
Anyway regarding your question, have to use a layered approach. In my experiences with my prior vehicle the undercoat is good until there's a breach. The undercoat then holds the moisture beneath. Address surface rust. Etching primer, primer. Thick layers. The 3M undercoat may be best. Then use the wheel well liners but make sure they're properly secured.
https://www.feoa.net/threads/upper-strut-mounts-lasting-12-months.117467/post-1123300
Much of the ass-end of my prior car was rusted out. But no one could tell just looking at it because the OEM undercoat was still intact. I could tell by knuckle-knocking around back there with my fist. Oxidation processes accelerate when the materials are exposed to higher concentrations of oxygen. (such as liquid H²O) Heat also accelerates the oxidation process.
