3D Print sanding/priming

[Aer0]

Hi, I'm new here but Ive had my 3d printer for a while and figured id finally get around to trying to make some props with it. To start, I'm going to make an Iron man helmet, such is the right of passage.
Ive made a small test piece to practice finishing the raw prints, and was looking for some input from people who have some experience.
My first real question is how do you know when to stop sanding?

So far on this piece i have:
Sanded 180, 240 grit
3 coats of filler primer
sand 240 grit
3 more coats of primer
wet sand 400 grit

This has left me with a smooth to the touch finish, but in some areas Ive sanded straight through to the plastic again, and you cannot feel it, but the layer lines are visible in some areas. Is this an issue, or am i good to prime it one last time and move onto paint?
Additionally, I have seen people mention using different coats of primer to identify low spots, but it seems i cant actually figure out how that method works. If someone is able to explain it that would be a big help.

Any advice would be greatly appreciated, Thanks.

 

[TazMan2000]

If you prime it again, you should be able to see the quality of the surface, if you have good light. If you are unsure, try taking it outside or into a brightly lit room to inspect. With other prints I wouldn't worry as much, but with an Iron Man you have to be extra vigilant, especially if you are painting the gold and chrome finishes.

TazMan2000

 

[DiggsBarklightr]

Depending on your wall thickness as you don’t want to burn through it, you can always filler prime it again and use a lower grit paper on it to smooth any of the print lines out. I’d go 120 and see what happens.

 

[ScoobiJohn]

i started using an epoxy resin over the print before sanding now - fills in most issues and is easier to sand back to a nice finish and adds some strength to the print and is a hell of a lot faster to get to a good result - that said - what you've done will almost certainly be fine once you give it another coat of primer then as tazman says inspect in good light and use some spot filler if any little bits needing attension

 

[Chimera59]

You can work with wood filler too instead of putty. I tried and it works pretty good too

 

[Aer0]

I took some of the suggestions on board and ended up with a pretty nice finish in my opinion, I then went and applied a coat of regular non-filler primer to prepare it for paint. Unfortunately the stuff i used turned out to be expensive garbage and gave me a horrendous surface texture no matter what i tried.

Is it even necessary to use a regular primer on top of everything? Or can i paint straight onto the sanded filler primer and have it turn out alright.

 

[Duncanator]

You can paint right on filler primer. You'll want to smooth it of course, otherwise you'll get the grainy-ness of the spray, but that's the case with any primer.

 

[Aer0]

I see. I was under the impression that using "regular" thin primer ontop of my sanded surface was the way to go. Am i going to run into any adhesion issues if small areas are sanded back to the plastic?

 

[TazMan2000]

I've used wood filler with some success. A person on YouTube recommended regular spackle. I've never tried it but an wondering if it will behave much like wood filler. Definitely cheaper and easier to sand. It seems to dry quicker as well .
Not so much worried about rigid pieces, but helmets that can flex a bit may cause flaking.

TazMan2000