Coating / Painting / Decal

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Pre-Treatment – Surface coating

The purpose of surface coating is to remove the unwanted surface impurities such as rust, dust, weld, etc. to ensure final paint quality.

There are 5 steps in this stage:

  1. Preparation of work load –material, quantity and size of the load are confirmed, and loaded into a metal cage.
  2. Rust removal: grease is first removed using chemical degreaser.  Then, dilute sulfuric acid bath is used to remove rust.  After completion of rust removal, frame is dipped into a rust prevention chemical before oven drying.
  3. Surface treatment: use sand paper and other hand tools to remove excess welds, smooth the surface and edge, and remove grease remains.  Also in this stage, all the threads on the frame are checked for possible defects.
  4. Coating: before zinc phosphate coating agent is applied, the frame is dipped into dilute sulfuric acid again for about 25 minutes to remove rust generated during step 3.  Then, oxalic acid is used to prepare the frame for zinc phosphate coating treatment.  The frame is then oven dried.
  5. Ready for painting.

Painting Process

  1. Dust inspection: Before painting process starts, sand, dust and any unwanted debris on the frame has to be removed. Otherwise, undesirable small bumps will show up on the finished work.  The worker slides over the frame surface with gloves to inspect for possible dust while strong air stream is blown.
  2. Base coating: the purpose of a base coating layer is to bridge the frame with the face coating and/or to prevent rusting of the frame.  Therefore, base coating has to securely cover the entire frame.  However, it is possible that static finishing leaves some dead spots unpainted.  When it happens, manual make up with airbrush is necessary.  When using airbrush, viscosity of the paint, pressure and distance of application all affect the quality of make up finishing.
  3. Still standing: the purposes of still standing are to enable even out of coating surface and the volatilization of solvents.  If the standing time is not long enough, bubbles and pin holes may be easily generated.
  4. Baking: baking evaporates solvents and enhances hardening.  Baking time, temperature and the heat distribution in the oven affect the quality of final results.  Normally, base coating is baked for about 6 minutes at 90 to 110 oC, middle coating for 18 to 25 minutes at 140 to 150 oC.
  5. Filling: when scratches or unwanted wrinkle on the surface are observed, filler is used.  Filler is normally applied after the base coating is completed.  Filler is better avoided as it may reduce the adhesion of subsequent coatings and increase bubbling.  If filler is used, further sanding and dust removal processes are required.
  6. Sanding: sanding is to remove unwanted particles on the surface and to smooth out the surface.  Sanding is an important process between two paint coatings.  It is a pre-requisite for a high quality coating in the next layer.
  7. For middle layer and top layer coating, and even the outmost clear lacquer coating, steps 2 to 4 are repeated.

Decal Application

In most cases, frame painting is not completed without decals.  There are a few different types of decals suitable for different applications.  For most high quality frames, heat resistant solvent transfer (HRST) and/or heat resistant water transfer (HRWT) decals are the ones of choice.

Application of these two types of decals is very similar.  The only difference is the solvent used to soak the decals.  The former uses butyl celusolve and water for the latter.  For both of them, decal is first soaked in the solvent, and the protective sheet is removed.  Then, the decal is positioned on the frame, and the base paper is removed.
Oven dry, clean, and then apply clear coating if necessary.