just a thought–you might get a spray can of Rustoleum plastic paint/primer to apply to the mushroom before the interlux brightsides paint. having said that, not all plastics are created equal, and plastic paint/primer may not stick to this material. the good news is that I found a source of the same manufacturer of mushroom vent on ebay (from an RV supplier in RV heaven in Northern Indiana) for about $12/free shipping so if you do a trial on one of your vents and it fails you can replace it cheaply.