hi, I wanted to paint the surface of a FDM style 3D printed part, and I wanted the pieces to have a smooth surface. I'm assuming that sanding won't work. I was wondering is some type of material could be added on top and then sanded down to create a smooth surface to paint on, and if anyone knew a process for how to do this. Edit: the pieces are square with large flat surfaces, so matching contours isn't a big issue for me.
http://www.shapeways.com/forum/pdf.php?th=19949& There's a couple of products - "Mr Hobby Mr Dissolved Putty" and "Mr Surfacer 500" Both are essentially Tamiya/Mr Hobby style putty's that have been mixed with thinner to create a liquid - with a consistency like cream. Use an old brush and slather it on, wait for 24 hours to cure then sand off. It's a lot softer than 3D printed plastic so it sands away easily. I've been using this on a couple of Shapeways "White Strong and Flexible Polished" kit's and it works well. You may have to do two or three applications and sandings to get it really smooth, and wet sanding seems to work best. It's a pain on fiddly, detailed parts as you basically end up filling in surface details, but for smooth flat surfaces it's the bomb.
There are videos on YouTube on smoothing 3d printed objects by letting them sit in acetone vapor. I've never tried it but I imagine it would cause sharp edges to smooth and you might lose some of the finer details.
This stuff works wonders, plus it's super easy to use: Amazon.com: Citadel Technical: Liquid Green Stuff (2012): Toys & Games
It’s probably not necessary for your specific project, but FYI, another great option for smoothing FDM print lines is xtc-3d by Smooth On. XTC-3D®, High Performance 3D Print Coating
Acetone only works if you are printing with ABS (Which most people don't). A decent car filler/primer and some sanding is probably your best bet - it's what a lot of the cosplay people use. Video
I recently used Vallejo Plastic Putty on some guns for Sixshot. That stuff is excellent for smoothing rough surfaces. I even use it for sculpting edges where the putty leaves a little bit of a raised surface.