What type of imperfections do you have? Air bubbles in the filler? filler at different levels? Is is a filler issue?
Filler, always use more than you need it shrinks , never sand with lower than 100 grit, get it in the ballpark with 100 but wider than the space you wanted to fill.
Go to 220 on a flat surface if you want it to turn out flat(no rises) leave enough past the edges to use 300 grit,until satisfied.
There is spot putty for air holes that may show up, use sparingly, sand with 300 on a sponge lightly until satisfied.
Filler type /sandable primer 3 coats or so ,wet sand with 400grit rolled over a sponge and use water, if sqweeged it should come up flat ,the shiny spots are the wet spots( lower surface) can be reprimed if shallow enough and resand smooth with 400 around a sponge, you can only build it up so much before it'll give you trouble so don't do it more than twice. Wet sponge it clean and let it dry . wipe it again before painting it just to be sure it's clean. use gloves keep skin oils off of it.
Best to paint the tunnel first when it's hanging.IMPORTANT use the same manufacturer and grade /type paint for each color on your 2 tone for compatabilities sake. numberous light even coats/no drips.let dry for 48 hrs
tape. 1/8" green tapefor edges- easy to bend-very sticky- won't allow bleed through. fill in with blue painters tape (don't soak it in paint ,can bleed thru). shoot the 2nd coat the same way as 1st, depending on design,you may want to remove the tape for a clean line before it's dry ,so be sure if that's the case that you leave a key in the tape that you can easily get and peel so you wont have to dig around wet paint with a razorblade to get an edge to peel. Take the tape off carefully and leave it all alone for 24hrs. then clear coat many even coats. give it a week wet sand with1200 grit and buff it out. done. be sure that the paint is not touching the rubber on your cap or the gas will get behind it and lift it. Use quality paint-it's just too much work to go through to use cheap paint. Good luck. This is what I do , I'm sure that there is other ways.