I found that this worked to remove most, not all, yellow urine staining on the underside of a white plastic loo seat.
1. Lakeland's OvenMate, which is a potassium bicarbonate gel. Paint on with brush provided, leave for 30 mins or more. Wipe and wash off. Obvio
usly take care - don't get it on your skin etc.
2. Finish with T-Cut (a mild abrasive for paintwork) to polish.
5 year old post, you taking the P.?