I don't trust those plastic inserts. The problem is you don't know what is happening behind them. In damp conditions water is going to settle between the plastic and metal and do its rusting in secret.

I found that happened with the front fenders of an '84. Where the plastic touched (clipped onto) the fender arc, was where the rusting was worst. So now I go without them and have clear access to hose down the inside and the area dries out faster.

The same problem occurs with undercoating. It hardens, separates from the metal, forming a crust under which water can get and cause problems. Undercoating should remain soft and sticky to be effective.

Yep, Ive got problems with the rear wheel wells and unlike the front fenders, you can't remove and work on them.
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.