Smooth any print with the guide coat method

The goal of this stage is to turn a rough 3D print into a smooth, paint-ready surface. The process is simple: apply filler primer, dust on a contrasting guide coat, and wet sand until the guide coat disappears. The guide coat remains in low spots and imperfections, making it easy to see exactly what still needs work.

If the guide coat remains after you’ve sanded through to the plastic, you’ve found a low spot. At that point, either continue sanding to level the surrounding area or fill the defect with spot putty. Once the repair is complete, re-prime and repeat the cycle.

Most parts require two to three rounds of primer, guide coat, and sanding. When the guide coat sands away evenly and the entire part is covered in a uniform layer of primer, the surface is ready for paint.

Tools & Materials

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Wet sanding station

Set up once and used every round. Full build in the wet sanding station guide once it publishes.

Prep for primer

High-build filler primer does most of the heavy lifting during the smoothing process, filling minor layer lines and surface texture as it builds. Before spraying, the goal is simply to give the primer the best possible surface to adhere to.

Mount the part so every surface can be reached without handling it during spraying. Then remove any dust, sanding residue, or grease that could interfere with primer adhesion or finish quality. Once the surface is clean, avoid touching it with bare hands until priming is complete.

1. Mount the part. Secure the part to a paint stick, holder, or stand that allows access to every surface during spraying. The holder supports the part while the primer or paint dries, allowing it to be moved and stored without touching the finished surfaces.
2. Clean the surface. Wipe the part with a lint-free cloth and a surface cleaner or degreaser. Pay special attention to recesses, panel lines, and areas that have been handled during assembly and sanding. Any remaining dust can become trapped in the primer, while grease and skin oils can interfere with adhesion.
3. Tack off right before you spray. Use a tack cloth immediately before priming to remove any remaining dust from the surface. Do not scrub or wipe the part aggressively with the tack cloth; tack cloths are designed to capture dust, and excessive pressure can transfer residue onto the surface. Instead, use light compressed air to dislodge dust while holding the tack cloth near the airflow to catch the particles as they lift away. The goal is a clean, dust-free surface, not to use the tack cloth as a cleaning rag.

Optional: Adhesion Promoter

Adhesion promoter is only needed on bare plastic before the first primer coat. If the part is already primed, scuffed, or being re-primed between guide coat rounds, skip this step. Promoter does not go between primer coats.

Apply one light, even coat from 8–10 in (20–25 cm) away and let it flash according to the label before priming.

Apply Filler Primer

Filler primer creates the sanding surface for the guide coat method. It fills minor layer lines and surface texture while giving you a uniform base to block sand.

1. Spray in light passes. Start with a light mist coat, follow with a medium coat, then finish with a wet coat if the surface is building evenly. Keep the can moving and overlap each pass by about half.
2. Rotate the part as you spray. Keep 6–8 in (15–20 cm) of distance and avoid lingering in one spot. Heavy buildup causes runs; spraying too close can also cause runs, while spraying too far away can create orange peel because the primer droplets begin drying before they reach the surface.
3. Let it cure before sanding. Most filler primers need 24–48 hours before wet sanding. Check the label. If the primer is still soft, it will gum up the paper instead of sanding cleanly.

Dust on a Guide Coat

This is the step that makes the method work. A light dusting of black sandable primer over the gray — the contrast marks every low spot, so your sanding progress is visible instead of guessed at. You only need a speckle, not coverage.

1. Hold the can 10–12 in (25–30 cm) away and fog a very light dusting.
2. Let it flash a few minutes. Once it is dry to the touch, you can start sanding.

Block-Sand at 400, Wet

Wet sanding keeps the dust down and keeps the paper cutting clean. Use a hard backer wherever possible. Fingertips follow waves in the surface; a block cuts them flat.

1. Soak the sandpaper for 5–10 minutes before sanding.
2. Start with the innermost edges and details. Use files or other shaped backers to sand panel lines, recesses, tight curves, and other hard-to-reach areas first. This prevents larger sanding tools from rounding over details before they are properly leveled.
3. Work outward toward the larger surfaces. As you transition from edge and detail work to broader areas, feather the sanding marks outward and blend them into the surrounding surface rather than leaving distinct tool marks behind.
4. Sand in straight, controlled strokes. Work across the layer direction when possible. Use a sanding block on broad surfaces and wrap sandpaper around a paint stick for larger flat areas to keep them flat and even.
5. Rinse often. Keep the part and paper clean. Clogged paper slows the process down and reduces the cutting power of the abrasive.
6. Read the guide coat. Areas that sand flat will lose the guide coat first, while depressions and surface defects tend to retain it. Sand until the surface is mostly uniform, but do not keep chasing a low spot once you start exposing plastic around it.

If plastic appears while guide coat remains in the surrounding low area, stop and decide whether to level the area further or fill the defect with spot putty. Either way, that area will need primer and guide coat again before moving on.

Spot Putty Touch-Ups

Sanding reveals what the primer couldn’t fill: pinholes, deeper scratches, the occasional seam. Thin putty passes handle them between rounds.

1. Spread a thin layer into defects only. Press it in with a plastic spreader at a low angle. Fill the void; don’t build a mound.
2. Let it cure fully (often under 30 minutes — check the tube).
3. Wet-sand flush with the surrounding primer. Use a hard backer here too, or you’ll dish the repair.

Repeat the Cycle — What Each Round Looks Like

One pass is never the whole job. Re-prime (or spot-prime), re-dust the guide coat, and sand again. Here’s what to expect each sanding round:

• Round 1: Tons of layer lines while sanding — this is where you can be the most aggressive to flatten the surface. If you burn through to the plastic, it’s not ideal but not a big deal at this stage. Try to leave as much primer as possible.
• Round 2: Mostly orange peel and minor flaws. It’s possible to finish a part with two rounds — try to avoid burning through to the plastic.
• Round 3: Touch-ups — filled blemishes and any burn-throughs from round 2. Hero pieces get this round; background parts often don’t need it.

Checkpoint: Paint-Ready

FAQ

Do I sand the raw print before priming?

Prior to primer, you want to shape the raw surface and deal with any seam lines, support scarring, etc. — but you don’t have to sand the raw plastic to remove layer lines.

Can I skip the guide coat?

You can, but then you are sanding blind and it comes down to guesswork. A guide coat visualizes progress and saves time.

What grit should I wet-sand with?

400 for most smoothing. If you are not planning to use a primer sealer, check your product data sheets to see what grit is best (usually 600–800) and finish the last round of sanding with a pass at that grit.

How long should primer cure before sanding?

Typically 24–48 hours depending on temperature and humidity. Always follow the label — uncured primer gums the paper and smears.

Does this work on PLA? Resin?

Any material. We print armor in PETG because it’s more heat tolerant, but the smoothing process is the same for PLA, ABS, and resin — resin usually needs one round instead of two or three.

How many coats of primer total?

Typically three passes — mist, medium, wet. Two to three coats per round, repeated each cycle as needed. Trying to bury a defect in one heavy coat usually just gives you a run, and as my painting mentor told me: runs are no fun.

Where you are in the series: this is Stage 03 of the Armor Finishing series. It picks up from Stage 02: Print Prep — supports, joining & filling and hands off to Stage 04: the paint stack (in production — coming soon). Keep your part mounted on its work holder — it stays there through paint.