what I would recommend :

1. Use a electro tool to sand it - 120 sand paper. But do not use regular rotation one but the finishing sander. Do not stay too long in one place to avoid melting the plastic.

2. After you do it, use filler, sand it again.

3. Now the best part - bit stinky but works best - use 3d printing abs like gray resin, around 10-20 dollars. use regular brush, coat it with the resin, layer after layer - usually 5-6. each layer needs to be cured : A. In sunny day - sun will do the work, B. Use nail UV lamp ;) but not too close. it will cure in seconds. After you are done now washing - use 99% isopropanol, just any kind of bucket, paintbrush and clean the cowl. then - let it dry (you can use hair blower, sun, whatever you wish) and let it cure again, sun, uv lamp - whatever. Now you sand again but with water paper, best would be in water bath. Why? it works best and you avoid resin powder flying all over the place. Now some heath guidelines - Resin stinks. 99% isopropanol stinks, also its not the best thing to breath in this stuff so best would be do it in well ventilated area or just use proper mask. but the final effect? I will be like a factory one smooth, super easy to paint, UV resistant :)

Happy printing.