Hello! What’s the best way to prime a 3d print (which material would be best) and is it true that car/mechanic varnsih is the best to make a finish coat on top of your paint once you’re done? Thanks!! (oh and also, do you have some tips to figure out if the seller who’s printing you something is overcharging you?)

If by prime you mean prepare the print for paint, then that is also entirely dependent upon the kind of finish you’re hoping for (mat/rough vs smooth/gloss).  For most props you will want to sand the print until the layer lines are relatively smoothed out.  If you want to make it look like it was not printed at all and more realistic, you can take the extra time to use fine automotive body fill.  These generally come as a 2 part mix where one is the putty and the other is the hardener.  After you’ve applied the layer you will need to sand again.  Sanding will definitely be the bulk of your effort depending on how fine you want it to be.  Filler primer works well as well if you’d rather get it close but do not need it perfect and in some situations can take the place of bodyfiller.

With regards to what you are using as a finishing coat, that again depends not only on the finish you want but the materials you’ve used before.  Personally, I’m taken to airbrushing a lot of my pieces so I use an airbrush clear coat and do several boats.  For a high polish finish you’ll want to make sure you either wet sand or buff as you go to keep it smooth and get out any imperfections.  Varnish can work but depending on the materials you use, it might react badly.

With regards to the whether someone is overcharging you for a print there are several variables that go into consideration.  Material wise, you will always be overpaying as depending on the infill % not as much material will be used as most expect, however, what you are paying for is time.  There will be times that major prints can fail and have to be redone. Not only is this time consuming but can also eat away at materials as well.  The larger the print, the longer it will take and the more potential it has to fail.  You can try comparing across other resellers or even hubs orders but factors that go into consideration is how fast they’re promising you the print, the dimensional accuracy, the material its printed in, the resolution, how much work you’d have to put in to get it finished, etc.  I know that’s not really a complete answer but the best way to look at it is whether or not you’re willing to pay to get it printed for you as a base versus building it from scratch.
       
Example of prop that’s been bodyfilled:

Sanding and bodyfiller:
https://www.facebook.com/theAngelofSteel/photos/a.657554467623599.1073741840.502454656466915/1365340363511669/?type=3&theater

After Sanding and Body Fill and filler Primer:

Clearcoating/buffing and prepping prop for molding stage:

Cheers,
Angel of Steel
Direct Sales & Marketing @ Shop3D.ca
Propbuilder @  Heroes Workshop