After sheet metal repair, the surface may still have dents and bumps; painting directly will make these defects "show through". Body filler (putty) is the plastic filler used to level out these pits and hollows. However, if it is not applied well or sanded flat, it instead plants the seeds of paint defects. This article explains the key points of applying, drying, and sanding body filler.
I. What is putty (polyester filler)
Automotive body filler is a paste based on unsaturated polyester resin, mixed with fillers such as talcum powder and a two-component curing agent that reacts when combined. After application, it cures rapidly and is used to fill sheet metal dents, welds, and deformations. It is merely a leveling layer; rust prevention still relies on the primer and electrophoretic coating, and it cannot be used as a primer.
II. Key Points for Knife Coating
- Substrate cleaning: Before scraping, be sure to remove oil and rust, otherwise the putty will not adhere.
- Mix thoroughly: Blend the curing agent in proportion (usually 2%–3%); too much causes cracking, too little prevents drying.
- Thin coats, multiple passes: Each coat 1–2 mm, let it dry thoroughly before applying the next layer; avoid applying a thick coat all at once.
- Edge thinning: Scrape the edges thin and feather them to avoid "steps".
III. Drying and Sanding
At room temperature of 20–25℃, body filler can be sanded in about 20–40 minutes, and higher temperature accelerates curing. For sanding, dust-free dry sanding is recommended as the priority, as it is efficient and does not cause moisture rebound.
| Method | Advantages | Precautions |
|---|---|---|
| Dry sanding (dry sander) | Fast, no moisture, fewer paint defects | Need dust collection, prevent inhaling dust |
| Wet sanding | Smooth, simple equipment | Must dry thoroughly, otherwise rusting/bubbling |
Summary
Putty is used to fill and level sheet metal defects; it should be applied in thin coats with multiple passes and dried thoroughly, with dust-free dry sanding preferred, to avoid sagging, pinholes, and later blistering caused by thick application. Clean before application, and remove dust and degrease after application before applying primer.
Frequently Asked Questions (FAQ)
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