How do I sand joints without wrecking the paper and leaving scratches that show through the paint?

Use a two-step approach: shape first, finish second.

Step 1: Shape the joint

• Start with 80–100 grit on a random orbital sander or sanding block.
• Sand until the joint is flush and smooth, removing ridges and high spots.
• Keep the sander moving—don’t dwell in one spot. This prevents gouges and heat buildup.

Step 2: Refine the surface

• Switch to 150 grit to remove the deep scratches left by the coarse paper.
• Follow with 180–220 grit for a smooth finish that won’t show scratches under paint.
• Always sand with the grain on the final passes, especially on wood.

Key tips to avoid scratches:

• Don’t skip grits. Jumping from 100 to 220 leaves deep scratches that finer grits can’t remove.
• Keep the paper clean. Dust clogs the abrasive and causes uneven scratching. Wipe the surface between grits.
• Use a sanding block or pad. It maintains even pressure and prevents dips or ridges.
• Check your work by running your hand over the surface or wiping it with a damp cloth—it’ll reveal scratches the eye misses.

Pro tip: If you’re sanding drywall joints, use a pole sander with 120 grit followed by 150 grit, then prime before painting to seal the paper and prevent flashing.

If you're working with paint-grade surfaces, 220 grit is usually sufficient before priming. For cabinets or furniture, go to 320 grit for a glass-smooth finish.

Use the right grit progression and technique—don't just "sand harder."

Grit Sequence (Drywall Compound):

1. First pass (knocking ridges): 100–120 grit (screen or paper). Use a pole sander with light pressure. Let the tool weight do the work.
2. Second pass (smoothing): 150 grit.
3. Final pass (paint prep): 220 grit (or 180–220 sanding sponge for angles). Never skip to 220 on heavy ridges—it loads instantly and scratches.

Critical Techniques:

• Lighting is everything: Use a raking light (headlamp or work light at 10–15° angle) while sanding. You’ll see scratches, ridges, and paper fuzz invisible under ambient light. Mark flaws with a pencil; sand only those spots.
• Pressure: Feather-light. If paper loads/clogs in 30 seconds, you’re pressing too hard. "Kiss the wall."
• Motion: Wide, overlapping arcs (pole sander) or circular (hand sponge). Avoid straight back-and-forth lines—they telegraph through paint.
• Clean between grits: Vacuum (brush attachment) then tack cloth or damp microfiber. Dust from 100 grit acts like 60 grit under 150 paper.
• Screens vs. Paper: Screens (100–150) resist loading on fresh mud. Switch to paper (180–220) for final finish—screens leave a subtle "corduroy" texture.
• Corners/Inside angles: Sanding sponge (fine/medium). Folded paper tears and digs.
• Primer reveals all: Apply a thin coat of PVA primer (or diluted all-purpose primer) before final 220 pass. It highlights every flaw. Sand after it dries.

Pro Tip: If paper loads fast, your mud isn't dry enough (wait 24 hrs+) or you're using cheap paper. 3M Pro Grade / Norton 3X / Mirka Gold last 3x longer than box-store generics.

Code/Standard: ASTM C840 (drywall finishing) implies Level 4/5 finish requires sanding smooth enough for "flat paint" – no visible tool marks or scratches under critical lighting.

Bottom line: Slow down. Light pressure. Raking light. Progress grits. Clean between. Prime to inspect.