Toyota Prado: Bull-bar deterioration

The problem you have is that deterioration of the finish is regarded as wear and tear, so to get a warranty claim up you need to show that the degradation of the finish on your bulbar is beyond what would be regarded as "fair" wear and tear. To do that you could collect photographs of other vehicles of a similar age that clearly show yours is worse and mount a case based on that evidence.   Ultimately though I reckon you won't be successful. I would recommend you go to a retailer handling truck accessories and get   their advice based on the actual condition of your bar.