Automotive body filler (unsaturated polyester putty, also sold as polyester putty) is a two-component filling material: the binder is an unsaturated polyester resin, viscosity is set by a styrenic reactive diluent, and cure is initiated by an organic peroxide. It levels dents, weld seams, pin holes and panel deformation before primer surfacer and the basecoat/clearcoat stage. The governing Chinese product standard is HG/T 4561-2013 "Unsaturated polyester putty", in force since 1 March 2014.
TL;DR
- Filler cures by free-radical copolymerisation, not by solvent release: the cured network is insoluble and infusible, so build thickness, hardener ratio and temperature matter more than air-drying time.
- HG/T 4561-2013 sets two grades. The gap sits in three mechanical limits: pull-off adhesion 3 MPa vs 5 MPa, impact resistance 10 cm vs 20 cm, flexibility 100 mm vs 50 mm.
- Conventional mix ratio is base : hardener = 100 : 1.5-3 by weight; the standard requires an open time of at least 3 min, while practical working time differs three- to four-fold between 10 °C and 30 °C.
- Through-dry is judged on the sanding disc: hand-sanding with P400 must not load the paper, with surface dry within 0.5 h and through dry within 4 h.
- Filler is not universal: heavy builds must be layered, waterborne basecoat must never be sprayed straight onto it, and repeated filler layers drive orange peel and sand-scratch show-through.
1. Composition and cure mechanism
1.1 Four functional parts of the base
- Binder: ortho-, iso- or bisphenol-A type unsaturated polyester resin, which fixes cross-link density and chemical resistance.
- Reactive diluent: styrene-type monomer that both thins the paste and takes part in copolymerisation; it is the main VOC source.
- Extenders: talc, calcium carbonate, precipitated silica and hollow glass microspheres, which control consistency, sanding behaviour and density.
- Additives: cobalt promoter, inhibitor (storage stability) and thixotrope (sag resistance).
1.2 Hardener and the reaction window
The hardener is a benzoyl peroxide (BPO) paste that, with the cobalt promoter, forms a redox initiation system curing at ambient temperature. The reaction is exothermic, and the core of a thick build runs hotter than the shop floor; that is the root cause of cracking, blistering and local overheating after a single heavy application. Cured filler conducts heat poorly, so assisted drying should raise temperature gently.
1.3 Where it differs from adjacent fillers
Epoxy fillers are tougher and shrink less but are harsher on the sanding schedule. Nitrocellulose glazing putties dry by evaporation, take little thickness per coat and are prone to pin holes. Polyurethane putties follow plastic movement better. Choose against substrate movement, required build depth and repair cycle time, not against hardness alone.
2. Reading the HG/T 4561-2013 limits
Test conditions are (23±2) °C and (50±5)% RH (GB/T 9278), sampling per GB/T 3186. Films are drawn at a wet thickness of (500±20) µm and conditioned 24 h before mechanical testing; flexibility and sanding use tin plate, impact and adhesion use steel panels.
| Property | Standard grade | Superior grade | Method |
|---|---|---|---|
| Consistency | 8-13 cm | GB/T 1749-1979 | |
| Open time (pot life) | ≥ 3 min | clause 5.7 | |
| Surface dry | ≤ 0.5 h | GB/T 1728-1979 method B | |
| Through dry | ≤ 4 h, no loading on P400 | clause 5.9 | |
| Flexibility | ≤ 100 mm | ≤ 50 mm | GB/T 1748-1979 |
| Impact resistance | ≥ 10 cm | ≥ 20 cm | GB/T 1732-1993 |
| Pull-off adhesion | ≥ 3 MPa | ≥ 5 MPa | GB/T 5210-2006, 20 mm doll |
| Sanding behaviour | P100 wet or P150 dry | clause 5.12 | |
| Storage stability | (80±2) °C / 24 h, no skinning | clause 5.6 | |
| Resistance to nitro lacquer | no lifting or blistering | clause 5.15 (30 min at 60-65 °C) | |
Selection follows the load case: sheet-metal repair, edges of plastic parts and vibration-exposed panels should use material meeting the superior grade; static castings or non-loaded joinery filling can use the standard grade. Ask for a type-test report covering adhesion, impact and flexibility (at least once a year under normal production) plus the nine factory-release items for each batch.
3. Cure window: ratio, temperature, time
3.1 More hardener is not faster work
Stay inside 100:1.5-3 and do not exceed 100:3. A lean mix gives long drying, a tacky surface and weak strength and adhesion. A rich mix gels before levelling, concentrates exotherm, and increases shrinkage and cracking while reducing bonding to metal. Cycle time bought with hardener is usually paid for in adhesion.
3.2 How temperature moves the clock
Working practice ranges, to be calibrated against each product data sheet: 10-15 °C about 5% hardener and 8-10 min open time; 15-20 °C about 4% and 6-8 min; 20-25 °C about 3% and 4-6 min; 25-30 °C about 2.5% and 3-4 min; above 30 °C about 2% and 2-3 min. The cold-shop levels sit above the 100:3 ceiling and are a trade-off that costs adhesion and low-temperature flexibility; a winter-grade filler plus booth heating above 18 °C is the better answer. Gel time for most products is 5-15 min, so mixed material is used once and paste that has already warmed up is never re-applied.
3.3 Drying and bake limits
Wet sanding works 0.5-1 h after application, dry sanding from 2-3 h. Where heat assistance is used, keep the part surface below roughly 80 °C and do not concentrate infra-red on one area. Residual moisture is a frequent source of later blistering and rust bleed-through, so wet-sanded panels need verified drying before spraying.
4. Application and sanding control
- Substrate: remove oil, rust and loose old coating, abrade to a continuous sound surface, degrease and apply immediately to avoid re-contamination.
- Layering: keep each pass at 1-2 mm where depth is needed and press the scrape to expel air; trapped air becomes voids and pin holes.
- Edges: use the final pass to feather the edge rather than to add film, which cuts sanding depth.
- Grit schedule: P80-P120 roughing, P180-P240 intermediate, P320-P400 finishing; glazing putty at P400-P600.
- Guide coat: spray a show-through coat before final sanding to reveal waves and low spots that flat light hides.
- Inter-layer cleanliness: sanding dust contains resin and extender; use tack cloths and the matching cleaner, since waterborne systems are especially sensitive to residue.
5. Defect-to-cause map
| Symptom | Typical cause | Corrective direction |
|---|---|---|
| Blisters, bubbles | One heavy build, excessive bake temperature, damp substrate | Layer thinner, keep below 80 °C, extend drying |
| Cracks, edge chipping | Hardener overdose, insufficient flexibility on thin gauge panels | Hold the stated ratio, move to superior grade or a flexible putty |
| Pin holes | Air trapped by light scraping, too-high viscosity, short flash | Press harder, extend flash, finish with glazing putty |
| Sand scratches under paint | Skipping grits, no glazing stage | Step down the grit schedule, add glazing and a guide coat |
| Lifting, colour bleed | Styrene swelling soluble layers of the old finish, missing sealer | Apply an isolating primer before filling, or a low-solvent system |
| Loading paper, slow dry | Lean hardener, cold and humid shop, poor mixing | Recalibrate ratio and booth climate, mix thoroughly |
6. System and regulatory boundaries
6.1 Never spray waterborne basecoat onto bare filler
A filler layer is porous and retains styrene and cobalt residues. Direct waterborne basecoat invites staining, rust flash and gloss unevenness. Build an isolating layer with primer surfacer or a sealer first; that layer also governs orange peel and film-thickness uniformity.
6.2 VOC and product compliance
Annex II Table B of EU Directive 2004/42/EC caps vehicle refinishing products by subcategory; body filler and stoppers are limited to 250 g/L VOC in the ready-to-use state, one of the tighter entries in that table. In China, coating VOC content is determined by GB/T 23985-2009 (difference method, IDT ISO 11890-1) and GB/T 23986-2009 (gas chromatography, IDT ISO 11890-2). Low-styrene formulations, styrene-free reactive diluents and high-loading extenders are the three routes, traded off against viscosity window, wetting and cost.
6.3 Safety and extraction
Styrene vapour density and mist fall-out drive local extraction design. Sanding dust and electrostatic discharge require the dust collection and explosion-protection measures set by painting-work safety codes. Keep resin and extender dust separate from metal dust.
7. FAQ
Is body filler the same as putty?
Body filler is the two-component unsaturated polyester putty that HG/T 4561-2013 names directly. Building wall putty is water-dispersed with a cement or polymer binder and has no place on moving automotive panels.
Does extra hardener save time?
Only on paper. Faster gel reduces cure uniformity and concentrates exotherm, so the standard adhesion limits (3 MPa standard grade, 5 MPa superior grade) no longer apply outside the specified ratio.
Why does the same paste behave differently in summer and winter?
The redox system is temperature-controlled, while the standard tests at (23±2) °C and (50±5)% RH. Dropping from 30 °C to 10 °C can triple open time; use a winter grade or heat the booth rather than doubling the peroxide.
Can filler go straight onto bare steel or galvanised sheet?
Abraded bare steel is acceptable. Galvanised sheet, aluminium and old finishes should first receive an isolating primer for adhesion and corrosion protection; zinc soap and white rust on galvanised steel sharply reduce pull-off values and must be removed.
How do I know the filler is dry enough for surfacer?
Use the standard's own criterion: no loading when hand-sanding with P400, reached within 4 h. The sanded surface should show no shiny wet patches and no residual warmth; wet-sanded repairs need extra drying time.
Is fibre-reinforced filler right for a bumper?
It adds strength and crack resistance but is comparatively rigid, harsh to sand and its dust is irritating. Polypropylene bumpers in flexing zones usually need a flexible putty, or plastic primer plus elastic surfacer, matched to the substrate and data sheet.
What paperwork should purchasing check?
Clause 7.1 of HG/T 4561-2013 requires the grade and the component ratio to be declared on the label or instruction sheet. Insist on a full type-test report from the last 12 months and the nine release items plus shelf life on each delivery.
Sources
- HG/T 4561-2013 "Unsaturated polyester putty", issued by the Ministry of Industry and Information Technology, administered by the National Standardisation Committee for Paints and Pigments, effective 2014-03-01
- Referenced test methods: GB/T 1749-1979, GB/T 1728-1979, GB/T 1748-1979, GB/T 1732-1993, GB/T 5210-2006, GB/T 1727-1992, GB/T 9278, GB/T 3186
- Directive 2004/42/EC Annex II Table B, vehicle refinishing VOC limits (body filler and stopper 250 g/L)
- GB/T 23985-2009 and GB/T 23986-2009 for coating VOC determination (IDT ISO 11890-1 and 11890-2)
- Published application practice for mix ratio, gel time and sanding windows from manufacturer data sheets
Related reading on this site
- How to use and sand body filler: applying, drying, wet and dry sanding - the hands-on application guide
- Automotive 2K clearcoat for refinish: low-temperature cure and flash point - the stage that follows the filler
Last updated: 2026-10-15 | Compiled by the technical team of Kexin New Materials (Guangdong) Co., Ltd. Limit values are quoted from HG/T 4561-2013 and public regulatory texts; application parameters must be verified against the data sheet of the product in use. Supply on EXW / FOB terms.