Re: Old dog learns new trick...Welding!
When you've sanded the plastic (Bondo) you'll have little pin holes. They are inevitible. I like to use a finishing putty. It has many brand names, I use 'Icing' made by USC. It is pourable and brushable and sands easily. It will fill all those minor imperfections and make the part ready for primer. I use a heavy fill primer too. It will also hide some of your sins. Sand, sand then sand some more starting with 280 or so, increasing the grit number down to at least 400 and even 600. Sand between coats, preferably wet sanded and then shoot the color sanding it between coats until you're ready for the clear coat.
Good luck. Keep us informed.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"