Finishing Putty: Types & Uses
Main types
- Gypsum-based (powder, interior)
Classic white skim for walls/ceilings in dry rooms. Sets fast, sands easily, very smooth. - Polymer/Acrylic ready-mix (interior; some multi-zone)
Pastes in buckets. Flexible, great workability, longer open time, low shrinkage. - Cement-based (interior/exterior, wet-prone)
Hard, moisture- and frost-tolerant; for façades, bathrooms (beneath waterproofing), kitchens, basements. - Lime/lime-gypsum fine finish (interior, breathable)
High vapor-permeability; good for heritage or where walls must “breathe”. - Lightweight/feathering compounds (spot repair/edges)
Ultra-easy sanding, for minor defects, joints, and feathering to zero.
When to use which
- Dry interiors (living/bedrooms/halls): Gypsum or ready-mix polymer for the best smoothness and speed.
- Wet areas / exterior basecoats: Cement-based (then waterproofing/paint per system).
- Breathable substrates (lime plasters, heritage masonry): Lime/lime-gypsum.
- High movement or temperature swings: Polymer ready-mix (better flexibility).
- Quick repairs, blending edges, last pass: Lightweight/feathering.
Key differences (at a glance)
- Moisture tolerance: Cement ▶ Polymer ▶ Gypsum ▶ Lime (lime is breathable, not water-resistant).
- Workability/open time: Ready-mix Polymer ▶ Lime ▶ Gypsum ▶ Cement.
- Hardness/impact: Cement ▶ Gypsum ▶ Polymer ▶ Lime.
- Sanding ease/finish: Gypsum/Lightweight ▶ Polymer ▶ Lime ▶ Cement (hardest to sand).
- Breathability (vapor): Lime ▶ Gypsum ▶ Polymer ▶ Cement.
- Layer thickness (typical skim): 0.5–3 mm (check data sheet; cement/lime tolerate slightly thicker passes than polymer/gypsum).
Application notes
- Substrate prep: Clean, sound, primed; fill >3–5 mm defects first.
- Setting vs drying: Gypsum/cement set chemically; polymers dry by evaporation—watch humidity/temperature.
- Sanding & dust: Use fine grits; prefer dust extraction; many ready-mix/lightweight sands with less effort.
- Paint compatibility: Most interior acrylics sit well on gypsum/polymer; use primers on cement/lime to control suction.
Pros & cons
Gypsum (powder)
- + Super smooth, easy sanding, fast setting, cost-effective.
- − Not for persistent damp; avoid direct wet zones/exteriors.
Polymer/Acrylic ready-mix
- + Great workability, flexible, low pinholes/shrinkage, long open time.
- − Heavier buckets to handle; longer drying in cold/humid rooms.
Cement-based
- + Moisture/freeze resistant, very hard surface; suitable outdoors.
- − Hard to sand; shorter workable window; can show trowel lines if rushed.
Lime/Lime-Gypsum
- + Breathable, mold-resistant by alkalinity; beautiful matte finish.
- − Slower set, lower hardness; needs skilled troweling.
Lightweight/Feathering
- + Ultra-easy sanding, perfect for edge feathering and last pass.
- − Not a primary build-up layer; limited thickness per pass.
Cost guide (percentages only; Gypsum powder = baseline 0%)
(Typical like-for-like smooth finish; actuals vary by brand, layer count, and site conditions.)
- Materials per m²:
- Polymer/Acrylic ready-mix: +20–60% vs baseline
- Cement-based: +10–35% vs baseline
- Lime/Lime-Gypsum: +15–40% vs baseline
- Lightweight/Feathering (for final pass areas only): +30–70% vs baseline (per m² of treated area)
- Labor impact (time/effort to paint-ready):
- Polymer ready-mix: −10–25% vs baseline (easier finish, fewer defects)
- Cement-based: +10–30% vs baseline (harder to finish/sand)
- Lime/Lime-Gypsum: +5–20% vs baseline (longer set, artisan finish)
- Lightweight (final pass only): −5–15% vs baseline on punch-list/defect stages
- Rework risk (pinholes, scratches, telegraphing):
- Polymer ready-mix: −10–25% vs baseline
- Cement-based: +5–15% vs baseline (if rushed/under-troweled)
- Lime/Lime-Gypsum: 0–10% vs baseline (depends on skill)
Risks & how to avoid them
- Moisture mismatch: Using gypsum in damp zones → softening/blistering. Fix: use cement-based in wet/exterior.
- Poor adhesion/dusting: Skipping primer or over-polishing substrate. Fix: prime per system; avoid glassy surfaces.
- Cracking/shrinkage: Layers too thick or forced drying. Fix: thin coats; stable ambient conditions.
- Drying delays (ready-mix): High humidity or cold sites. Fix: ventilation, gentle heat, longer cure before paint.
- Powder contamination/lumps: Bad mixing. Fix: clean water, correct ratios, rest & re-mix (slake) per data sheet.
Quick choice guide
- Most interiors, fast & smooth: Gypsum (powder)
- Premium finish, fewer defects, flexible: Polymer/Acrylic ready-mix
- Wet rooms/exterior base & high hardness: Cement-based
- Breathable/heritage walls: Lime/Lime-Gypsum
- Final feathering & spot fixes: Lightweight/Feathering
Short FAQ
- Can I skim directly over paint? Yes, if the paint is sound and primed; otherwise strip/abrade.
- Over cement render in a bathroom? Use cement-based skim, then waterproofing/tiles or moisture-rated paint.
- Underfloor heating walls (radiant panels)? Prefer polymer or gypsum rated for that application; manage temperature/humidity.
