Paint prep: support removal, joining and assembly, and dealing with major blemishes

This stage prepares the raw model for primer. It's not about layer lines yet: it's about locking in the core shape. If a multi-part print will be painted as one piece, glue it together; where parts join through seams, plastic welding does the job. Deal with major blemishes with filler, and leave the layer line removal for Stage 03. This is the one stage where you can be assertive with tools; minor scratches will be buried under filler and primer later.

When you're done, a primer-ready part will:

• Have all support material removed.
• Be permanently joined (if multi-piece) with seams flush.
• Have major defects flattened.
• Have the surface prepared for primer and paint.

Tools & Materials

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Remove supports

If your support settings are dialed in from Stage 01, supports come away easily; the video shows what that looks like.

1. Start with the large pieces. Break the big support structures away first.
2. Cut remnants close. Where supports leave material behind, go in with flush cutters and get as close to the surface as you can. You can pry with them too; just be careful not to rip away the actual model.
3. Separate fused interfaces. Sometimes a hobby knife is what it takes to get in and separate the support interface from the part itself.
4. Knock down the nubs. Smaller support nubs are better dealt with on the raw print than under paint later. Use needle files or similar to knock the raised bumps down.

Join multi-part prints

If the print came off the build plate as a single piece, skip ahead to filling.

For larger props, you'll usually have multiple sections that need to join into one solid piece. The goal here is to get the parts into alignment, bond them with the right joining method, and prep the seams so they disappear once you start filling.

1. Dry-fit first

Before any glue or welding tool comes out, put the pieces together exactly as they'll be assembled. Something like painter's tape works to hold the pieces together while you check the fit. Check how the edges meet, look for gaps or overlapping layers, and see whether the parts naturally align or fight each other.

2. Correct the fit

If the parts don't line up during the dry-fit, don't force the joint. Diagnose first, then correct the cause:

• Inspect the mating surfaces. Burrs, support remnants, or high spots cause most of the misalignment. A quick pass with a hobby knife, flush cutters, or a needle file removes the interference.
• Trim and sand. Carefully trim any fused layers and sand the mating edges — start coarse, finish fine — until the faces sit flush.
• Flatten edges on a sanding plate. For a truly flat mating surface, glue a large piece of sandpaper to a flat surface, like a big board, and slide the part across it. Instead of hand sanding and hoping it comes out flat, you're pushing the part across a flat reference, and it levels the whole edge at once.
• Use shims or packing. Thin card, tape, or PTFE film can act as temporary shims to see how small spacing changes affect alignment and to hold geometry while you plan a permanent fix.
• Gentle heat forming. For slight warps, apply controlled heat to the distorted area (hair dryer or low-setting heat gun), then clamp to shape while cooling. Work slowly; over-heating can deform details or release fumes. Test on a scrap if unsure.
• Add mechanical registration. If weak registration is the root cause, drill small pilot holes and add alignment pins, dowels, or tabs to guarantee repeatable positioning during bonding.
• Clamps or a jig. Build a simple jig or use clamps with soft pads to hold parts precisely while you tack-glue or weld short sections.
• Plan for filling or reprinting. If the misalignment is minor and only cosmetic, proceed and remove the step later with filling and sanding. If the fit is structurally wrong — large gaps, mismatched features — reprint the offending piece with adjusted tolerances, registration keys, or a different orientation.
• Re-evaluate the model. Multiple alignment problems often mean the slicer orientation or scale needs adjusting. Fixing the model or slicer settings and reprinting saves time over fighting a fundamentally wrong part.

3. Choose a joining method

The joining technique depends on the material, the stress the joint will take, and how clean the seam needs to be:

• Plastic welding. Fuses the pieces into one solid part. The best method: use a soldering iron to cut a channel a little more than 50% of the way through the part along the seam, then fill the channel with fresh filament to build up a bead, and grind the bead flat. Flip the part over and do the same on the other side. That gives you 100% penetration of fresh filament joining both sides. Best for load-bearing areas.
• Cyanoacrylate (CA) glue. Fast, strong, easy. An activator speeds up the bond; fair warning, activator makes it instant, so have the parts aligned before they touch. Great for smaller parts or quick assembly, but can be brittle if the joint takes repeated stress. Buy it in small 3 g tubes: once a bottle is opened, moisture in the air starts curing the glue in and around it, so larger bottles usually go bad before you use them up. Small tubes you open and get full use out of. Baking soda works as an activator too, and it's good at building structure; you can use it to fill and bond at the same time.
• Two-part epoxy. Slower cure but extremely strong. Works well when you need time to position parts before the bond sets.
• Mechanical reinforcement. Hot staples, small screws, or pins add hidden strength in high-stress spots.

4. Bond the pieces

Work in sections if the seam is long. Apply adhesive or welding heat to a short stretch, join it, secure with tape or clamps, then move to the next section. This keeps alignment tight all the way through.

If welding, watch the heat. The joint should melt together, not collapse. For glues or epoxy, resist the urge to move the joint before the full cure time.

5. Reinforce if needed

Some props benefit from extra reinforcement — especially ones with moving parts or that get handled a lot. Embedding a hot staple along the inside of the seam adds significant strength without affecting the exterior.

For a plate that's been seamed together, back the joint with a coat of fiberglass resin and a layer of fiberglass cloth on the inside. It makes the panel very strong, and you don't have to worry about it cracking or breaking later.

Fill imperfections

Hide seams, nicks, and layer artifacts with thin, controlled filler that sands clean and feathers invisibly into the surrounding plastic. Several thin passes always beat one thick mound.

1. Prep the surface. Knock down high spots and loose fuzz. Wipe dust with a clean, dry cloth.
2. Tape for control. For deeper fills or seams, border the area with tape to limit spread and reduce sanding footprint.
3. Apply thin. Using a small spreader, press a thin layer across the defect at 45° to the seam. Fill the void; don't build a mound.
4. Feather edges. Pull the spreader long and shallow to blend edges into bare plastic.
5. Cure fully. Wait the manufacturer's time — often 15–45 min for glazing putty — until the putty is firm and non-gummy.
6. Block sand. Sand at 220 with a hard backer in straight, cross-hatch strokes. Light pressure avoids dishing the repair.
7. Reassess. Repeat thin fill and sand cycles as needed. Reopen any buried panel lines with a light scribe pass.
8. Finish to 400. A quick final pass scuffs the area for primer without polishing it slick.

Technique callouts

• Seams. Sand perpendicular to the seam with a rigid block so you don't round over the joint.
• Pinholes. Smear putty across the area, then scrape nearly clean. The voids stay filled without build-up.
• Engraved or raised details. Mask edges with tape before filling. Remove the tape before the putty fully cures to avoid a hard ridge.
• Rebuilding an edge. Tape off the edge with painter's tape, fill up to the tape, and let it cure. Pull the tape and dress the corner with something hard and flat, like a needle file. That's a good way to rebuild an edge lost to overhangs or another blemish during the print.

Selective sanding

Selective sanding is not about removing layer lines. It's about knocking back and shaping the defects on the surface before the primer phase starts. Rough, drooped areas also soak up more Bondo and primer than they should, so flattening them now saves filler later.

The big one is the seam line. As the layers stack up, the printer leaves a seam where the nozzle moves to the next layer: not a joint you made, just a ridge in the print. Flatten it now, while the part is bare plastic. Wait until after primer and the same fix means sanding back through the primer and into the plastic again, or building up so much primer and filler to bury the seam that you lose the actual detail around it.

You can be aggressive with tools here: files, hand tools, even power tools all have a place at this stage. Where to spend the effort:

• Drooped overhangs. Start with a sanding pass to flatten the worst of the drooping, then plan on Bondo after the first primer pass to finish the area.
• The printer's seam line. Knock the ridge down flush with the surface.
• Your repairs. If you welded a joint, ground it down, and filled it with Bondo, feather the repair out so you get a little flatness around it. You're not flattening layer lines; you're blending the repair.

1. Mark and inspect. Shade defects with a pencil — the marks act as a guide coat so you can see what's leveled and what isn't.
2. Start coarse, stay targeted. Use 120 only on the defect with a rigid backer. Keep strokes straight and cross-hatched.
3. Refine the area. Move to 220 to clear the scratch marks. Blend slightly beyond the defect, not the whole panel.
4. Protect edges. Use a hard block on edges and raised features. Light pressure, stop early; rounded edges are hard to recover.
5. Re-check. Repeat lightly as needed. If a low spot persists, return to filling instead of over-sanding.

Checkpoint: Primer-ready

That's the stage: supports off, parts joined into one piece, major defects filled and knocked back level. The philosophy behind all of it is locking in the core shape while the part is still bare plastic. Every one of those fixes is cheaper now than it would be under primer, and the smoothing and paint work ahead goes better when it's refining a surface, not reshaping a part.

FAQ

Can I skip the dry-fit on a two-piece print?
You can, but most alignment problems are easier to fix before glue or weld goes on. The dry-fit takes a minute and saves redoing a bonded joint that doesn't sit right.

What grit should I sand to before primer?
First, read the product data sheet for your filler primer or filler of choice; it will tell you what surface prep it needs. Generally, 220–320 is plenty for most filler primers, and they'll fill those scratches with no problem. Anything finer is wasted sanding, because the filler primer fills the scratches anyway. Anything coarser and you risk building up more filler primer to cover the scratches, and potentially losing detail. 220 is about the sweet spot.

Can I weld PLA?
Yes. You can weld any of the materials; each just may need a different temperature on the soldering iron.

Bondo or glazing putty: which is better?
Bondo is a brand name, and it covers both: a two-part structural mix and a glazing putty. The two-part mix is great for filling larger defects and deeper issues; the glazing putty is good for minor surface leveling and filling cracks and seams. Both can coexist on the same part.