SLA/DLP Resin 3D printing is ideal for producing tiny parts or miniatures with fine details and high accuracy. Resin 3D prints generally come with a smooth surface even without any post-processing.

But some imperfections are inevitable like horizontal layer lines,  marks and craters caused by the removing supports. Especially if you are about to paint your print, smoothing out prints before painting is essential because we all know painting hides no flaws.

How to Smooth Out the Print

There are several ways to smooth out 3D prints, such as sanding, smoothing with chemicals like solvents, or spraying with filler primer or combinations of these methods. But today we will show you how to smooth 3D prints with sanding, which is the easiest and cheapest method. Before you start sanding, make sure the support structures have been removed and get the tools prepared. Here is the tools checklist in this guide:
• Micro grit sandpaper (learn more about sandpaper grit and grade)
• Precision Knife
• Side cutter diagonal plier

The sanding process is simple and straightforward. To sand down a 3D print, just use different grits of sandpaper, from coarse to fine (the smaller the grit number gets, the sandpaper gets coarser), to remove the marks and achieve a smooth surface.

Step 1: Sand Down Marks

Beginning with P400 grit sandpaper, sand down support marks, layer lines, and those apparent blemishes. Sand evenly in circular motions or gently follow the curve of the object. The sanding process can be time-consuming, you probably want to sand the high spots hard to speed up the process. It’s not a good idea as it could cause additional marks and even damage your print. 

 
When sanding the edges, small cracks, or any small spots that are hard to approach, use a precision knife to cut off the bump, then use pliers to take it off,  and then continue sanding with a cropped sandpaper that can fit into the tiny places.

Step 2: Sand the Entire Print

When the obvious blemishes are removed, you will see some white scars or scratches on top of the surface left by sanding. To fix it, move to P800 grit sandpaper to sand the entire print gently. Feel the surface of the print and feel for bumps or unlevel areas. When you think it‘s smooth enough, use water to wash away the dust and wait for the paint to dry completely before going forward to the next step.

Pro Tips for Sanding

1. DO observe the surface once in a while when sanding, make sure you don’t overdo it
2. DON'T over sand one single area, it could cause marks that need extra sanding, or worse, damage your print.
3. DON'T be afraid of messing up, keep trying and you’ll get better every time.

How to Spray Paint Your 3D Print

After you've given the print a smooth finish, it's time to move forward to painting! Don’t get over-excited, we need some preparation first.

Preparation

1.  Tools Checklist

• Primer
• Paint
• Mask Tape + Cutter
• Drill + Stick
• Safety Equipment: Face mask, gloves

2. Mount the Object (Optional)

This step is not essential if you can paint all surfaces evenly with a handle on the model. We did not have one for this SainSmart logo model here so we drilled a hole on the bottom and decided to mount it on a small stick. Then we can just hold the stick when painting. After drilling, wash the model to remove any dust produced during the process, and let it dry.

3. Choose an Appropriate Location

The paint result can be affected by environmental conditions. Don’t paint when it’s humid or windy, choose a bright location where you can inspect the paint easily.

Step 1: Priming

Primer is a special type of paint that can stick to the object and ensures better adhesion for the paint to bond with the object's surface. It can also hide the base color of the printing materials. More simply put, primer prepares the surface to help make the paint color the best it could be! So prime when you can.

Choose Primer Color
Primer comes in colors too, it’s usually in white or other neutral colors such as gray. If you are choosing between white and gray, pick gray if your paint is gray, otherwise pick white. Theoretically, a lighter color is a relatively safe option compared to a dark color because it reduces the chance of blending with the topcoat color. Or you can just choose a primer color that is close to your topcoat color.

Apply Primer

Apply 2-3 layers for the primer, be sure to check constantly how the color adheres well to the surface. To see if it’s over-painting or do any areas need extra filling. When the base color is hidden,  let the model sit for 20 to 40 minutes to dry.

Step 2: Painting

When the primer is dry, use the same techniques to apply paints. Be sure to shake the can before use and paint in thin layers.

Using Masking Tape

To create a multicolor model, you can print it using multiple materials or split the print into parts, and color them separately with different paints. 

If you are applying multi colors to a whole model, divide it into parts by different colors. Like the SainSmart logo here, we divide it into 3 parts for painting:

• Eyes: White
• Goggles: Blue
• Body: White

Usually, we paint from a small area to a large one, so the painting order should be: Eye>Goggles>Body. When painting different parts,  use masking tape to cover those areas you don’t want to paint.

Be careful when cutting edges of the tape, make sure you’re tracking the borderline, and avoid bubbles to keep the paints from seeping over.

Wait for the paint to be fully-dried before applying another coat.

Step 3: Apply a Clear Coat

When the paints are dry, remove the tape to reveal all surfaces, do it slowly to prevent peeling off any paint underneath. To achieve a long-lasting paint job, finish with 1-2 layers of clear coat as protection. By choosing different finishes, you can also decide the look for the model, whether it’s matte or glossy.

Pro Tips for Painting

DON'Ts

1. DON'T linger too long on any area, it will build up uneven layers
2. DON'T touch the object with your finger when the paint is not 100% dry, it will leave fingerprints on the surface.
3. DON'T paint when the humidity is high. Too much moisture in the air can cause surfactant leaching (sources).

DOs

1. DO paint in a well-ventilated area.
2. DO shake the paint can quickly for 5-10 seconds before use to mix the paint inside.
3. DO make sure the paint is fully dry between coats.
4. DO spray a thin layer at a time, avoid overly-thick application.

The key to a perfect paint job is to spray evenly with thin layers and constantly inspect the surface. Then like what we said about sanding, keep trying and you’ll paint like a pro.

Hope you enjoyed this guide for finishing 3D prints! We'd like to thank our colleague Lance for giving advice on this guide and designing/printing the cute sitting SainSmart logo model.

Feel free to share your thoughts or techniques below this post, or simply reach the community by joining the 3D printer users group on Facebook.