This is a set of instructions and handy tips for prepping a flat or gently curved 3d print for painting, with the aim of a super smooth, mirror finish.Things you will need:Automotive Filler Primer400 grit and 600 grit wet&dry sandpaperPlastic primer (optional)Good lightingPatience1.Print with a layer height below 0.15mm. 0.2mm will work, but will require many more iterations of the filling/sanding process, so the time saved printing usually ends up being used sanding. Whose time is more valuable; the machine’s, or yours?Use at least three perimeters, and at least 5 top/bottom layers. Anything less and the sanding process will reveal holes in your print that you won’t be able to fill.Use a filament colour that will contrast against your chosen filler primer. This makes it much easier to tell when it’s time for the next coat.If you have a crappy printer like me and have to choose between zits from overextrusion and scars from underextrusion, choose zits. You can remove them with a knife or even a fingernail, but scars require many coats of primer (or even filler putty if they’re really deep) to fill.2.Remove any blobs and put on a thick coat of filler primer. Don’t sand the bare plastic before your first coat of primer, it’s too much work. Sanding will be much quicker and easier once you get some primer on the part.You need to use a primer marked “filler primer” or “high build primer”. Initially I bought plastic primer, which I later discovered is designed only to help paint adhere to an already smooth plastic surface, and has no gap-filling abilities.You can really glob the filler primer on for this first coat, it doesn’t matter if you get orange peel or even runs.3.Wet sand with 400 grit. Yes, 400 grit, and make sure it’s wet. Anything coarser will have you back to bare plastic before you even realise what’s going on.It’s very important to regularly wash the dust out of the sandpaper, and also regularly wash the dust off the print and inspect it under a powerful light.Make sure you sand perpendicular to the layer lines. The printed plastic, even if it’s PLA, will be much harder than the primer. If you sand along the layer lines, the sand particles will strip the soft, chalky primer out of the crevices between the layers, while leaving the harder plastic intact, making the whole exercise a net zero.When you see an even zebra-stripe pattern, with lines of primer between each layer, you’re done sanding this coat.4.Repeat the primer and wet sanding process until you’re happy with the smoothness of the part. Get a new piece of sandpaper for every coat, or change even more regularly if you have a large part to sand. Especially on the first few coats, when you’re removing a lot of plastic, the sandpaper will wear smooth very quickly.You can usually call it a day after three coats, but there will likely still be some very subtle blemishes from the deepest of your underextrusion scars or most poorly aligned layers, so if you want it perfect you might need four or five coats.After about the third coat, you’ll need to wash the part with water and then dry it each time you inspect it, as the voids will likely be so shallow that primer dust or water will hide them, even under powerful lighting.If you’re not sure whether to sand out a scar or apply another coat of primer to fill it, here’s a good rule of thumb: If you see primer at the bottom of the void, you should be able to sand it out. If you don’t, you should probably fill it by putting on another coat.5.When you’re happy with the surface, having sanded it back to an even zebra pattern, hit the part with one more coat of filler primer. If you have plain plastic primer, you can use it for this coat.You want a very thin and even final coat, mist it on like you’re spraying paint for a mirror finish. You’re not trying to build material onto the part any more, you just want to ensure that the surface is entirely coated with primer to help your paint adhere.6.Once the final coat is dry, very lightly wet sand it with some even finer 600 grit paper.Use small circular motions and sand very lightly and evenly across the whole part, so as not to expose the plastic underneath.Since you’re sanding pure primer, this won’t take very long at all. You’ll know you’re done when the surface feels super smooth and slippery http://ift.tt/2CD3iAJ