I have a K&N intake on papa smurf, and I noticed that the plastic was cracking where it mounts to the TB, so to fix it, I used plastic bumper repair glue (works great), and since I had to sand it, I got a bit carried away with the prep, which led to paint, and more sanding, and even more paint... LOL
Here is the repair:
the top side:
On the engine:
and the truck on the beach:
In case you wanted to know, I sanded from 50 to 150 grit pre-paint. and used Rustoleum Plastic Silver paint, then wet sanded it to 400 grit, 2 coats Rustoleum Plastic Clear, and wet sanded it again, with one final coat of clear...