Gillard...this one [8in] was painted with rattle cans. It is fiberglass so painting was a snap. Took 4 cans to prime and 4 to finish with 2 cans clear cost was 42.00 Krylon sized cans.
First I would need to consider the material being painted.
The quality of finish you wish to attain.
If it's cardboard or something porous like concrete casting tubes, I would seal it with thin down epoxy [if that's possible for you] it would reduce all the following sanding and filling and priming greatly. That's if you are going for a top shelf finish.
Or yes I have helped others with large tubes.Use a fair quality house type paint for priming and roll or brush it on. Lots of sanding if you want a smooth finish.
Then go to rattle cans to finish.[Takes way fewer cans to do it]
Best results with house paint where a "class finish" is not required" or as I like to say when viewing the rocket from at least 10ft away......looks good from here Were done with what we call airless sprayer mainly used for house painting.
You can do a pretty fair job easy and quick if you know someone with an airless spay gun.
If not go down to your local pub at quitt'n time and find some painters just off work and buy a couple of beers. They are easy to spot, paint on the cloths, or call local paint store to find out the local watering hole.
I know that's where all mine were after work, especially Fridays after pay. LOL Who painted your flat or house, give them a call.
Tell them what your doing ......[rockets].... offer them a couple bucks AND you will have BEER after the rocket is finished. You will probably get it done for free,in any color paint they have on the truck and in record time! LOL It would take me less than 20 minutes to set up, paint and be done on a rocket this size. Then you can do the prep for finish paint and have one over again for finishing it off.
Of course never offer ALL the beer you can drink as payment, it would be far cheaper to have Rolls Royce paint your rocket after paying for the beer consumed!
All kidding aside: But it is a viable option. I have been asked and done many small jobs from contacts at a bar...... for beer or trade.
Best results are with Flat and eggshell type finish[matte or very low sheen] house paint.
I used both my HVLP for wood work with thinned house paint. [Gloss]
That rocket came out beautiful....like cabinets.
But still had to do all the prep and sanding used for rattle or auto paint job.
The quality of the job will = the quality of your prep and skill regardless of materials used. And the quality of the tools used.
With a quality brush and great paint I have rolled [good roller cover, no cheepies] I have painted many things [doors, cabinets, boats, that looked like they were sprayed.