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Tech Tip: Fine Body Filler


John Hristias – PPG Business Support Manager Asia/Pacific

Smaller filler jobs and greater accuracy
Nowadays, the majority of body filler repairs are either completed or finished off with ‘finer’ grade filler products. While there is a need to build efficiency into the process, it’s also crucial to avoid bad habits that lead to costly and frustrating reworks. When tackling smaller repair areas, it’s even more important to have a precise, step-by-step filler method to ensure a quality result.
 
Tips and recommendations
  • Accurate hardener ratio – Hardener to filler ratio is always critical but precision is more difficult when mixing smaller volumes. Typically, the recommended hardener is just 1% to 3% of the filler volume. Unless it’s being weighed on scales, this is almost impossible to get exactly right – too little hardener leaves the filler under cured (causing shrinkage and rework), while too much leads to bleaching which stains through the paint film. Often shrinkage is blamed on primer filler when it actually came from inaccurately mixed body filler moving below the layers. A better option is a modern product that uses a dispenser system to ensure the filler and hardener is always accurate. For example, PPG’s STARTLINE® Ultra Body Filler with the handy Cartridge Dispenser system – the product remains sealed in the cartridge until used, it adheres to most substrates and its fine texture minimises pin-holes.
  • Mixing technique – The common method of folding filler onto the hardener again and again actually forces air into the mixture and creates pin-holes. The recommend technique is to pick the hardener up on the back of the mixing tool before pressing it into the filler using a side-to-side fishtail motion and repeating until a uniform colour has been reached.
  • Not over paint film – PPG doesn’t recommend applying body filler over existing paint film – it’s designed to expand in heat and shrink in cold which is not good for body filler.
  • Substrate preparation – Remove existing paint film with P80 abrasive about a hand-width around the repair area. This leaves a deep enough scratch pattern for the filler to adhere to. Featheredge the area surrounding the repair edges with P120 and then STOP! Sanding the area with something finer, like P180, simply polishes the surface and makes it hard for the filler to stick.
  • Wax & grease remover – After feather-edging, clean the repair area with wax & grease remover to eliminate dust and contaminants. These can prevent filler getting a good surface bond and cause blistering and peeling.
  • Application – Get fine filler application right and the extra step of pin-hole filler is often not required. Rather than trowel on a thick layer, start by pushing a tight, skim coat firmly into the metal of the repair area and then apply heavier, thicker layers over the top. Building with successive thin layers will minimise the risk of craters and pin-holes.
Dumb stuff – Stick to the recommended process and avoid making dumb choices, such as mixing heavy and light filler together thinking it’s going to be easier to apply. In particular, don’t mix two different brands of filler together or the hardener from a light filler into a heavy filler and vice versa.