Code of practice for the preparation and application of putty for repairing plywood and other wood-based panels
1987 Edition

Recommended formulations as per the standard include:

1. Nitrocellulose-Based Putty:

• Acetone (80 parts), Methanol (40 parts), Nitrocellulose 20 Sec (15 parts), Nitrocellulose 1/2 Sec (20 parts), Gypsum (60-90 parts), Talc powder (60-90 parts), China clay (20-30 parts), and Wood flour (15-20 parts).
• Mix solvents first, then gradually add nitrocellulose grades with stirring, followed by fillers to create a uniform paste. Pot life is approximately one week.
• Colour can be adjusted using mineral pigments such as raw sienna or ferric oxide mixed with wood flour.

2. Urea-Formaldehyde Based Putty:

• UF resin powder (100 parts), Wood flour (40 parts), Talc powder (100 parts), China clay (100 parts), and Acetone (120-130 parts).
• Powders are blended before adding acetone and stirring to form the putty base with about a one-week pot life.
• Addition of 2 ml of 20% ammonium chloride solution per 100 parts of putty base acts as a catalyst, reducing pot life to roughly two hours.
• Colouring employs similar mineral pigments and wood flour.

Suitable fillers include wood flour (75 microns), talc powder (45 microns), gypsum (45 microns), and china clay (45 microns). These fillers help replicate the texture and improve the putty’s physical properties. For color matching, mineral pigments such as raw sienna, burnt sienna, turkey amber, ferric oxide, titanium dioxide, and zinc oxide are recommended. The pigment amount can be reduced by adding wood flour, fine sand, or dust. These components are blended into the putty to achieve the desired shade and texture, ensuring close resemblance to the wood panel being repaired.

The standard specifies acetone and methyl alcohol (methanol) or ethyl alcohol as suitable solvents. For nitrocellulose-based putty, the typical solvent ratio is 80 parts acetone to 40 parts methanol by weight. These solvents must be pure and free from contaminants to maintain the putty's quality. The preparation involves mixing acetone and methanol, then gradually adding nitrocellulose grades with stirring, followed by fillers to produce a smooth, homogeneous paste. This solvent combination ensures proper workability and a pot life around one week.

Pot life refers to the timeframe during which mixed putty remains workable before it hardens or loses plasticity. According to the code, nitrocellulose putty generally has a pot life of about one week without catalyst addition. Urea-formaldehyde resin-based putty before catalyst addition also has approximately a one-week pot life, but once 20% ammonium chloride catalyst is mixed in, the pot life shortens to around two hours. Applying putty within its pot life is critical to avoid adhesion problems or cracking. It is advisable to prepare only the quantity that can be applied within this period for optimum results.

To achieve long-lasting, crack-resistant repairs, the following application practices are advised:

1. Prepare the surface by cleaning it thoroughly to remove dust, grease, and moisture.
2. Use well-mixed, uniform putty formulations free of lumps.
3. Apply putty in thin, even layers using a flexible metal putty knife, avoiding thick coatings.
4. Allow each layer to dry fully before applying the next.
5. Remove excess putty after drying with a scraper and sand the surface smooth.
6. Protect the repaired area from moisture during curing.

These steps help in obtaining a durable finish that blends well with the wood and resists cracking and water damage.