Carrozzeria
 

PAINTING PROCESS ALUMINIUM

Many years of experience show that the behaviour of coatings on aluminium depends on the surface preparation ,
Applying paint to an aluminium alloy surface involves a number of steps:

Degreasing

The purpose of degreasing is to remove all foreign bodies, including solid particles (swarf, dust) and fatty products (oils, greases), which have infiltrated the metal's natural oxide film.
Degreasing with detergents

(Teepol or equivalent type) is preferable to the use of organic solvents . Solvents that are too "light" such as acetone are not recommended as they are tricky to handle and highly flammable.
Degreasing should be done by treating small areas at a time, using clean lint-free cloths that are frequently replaced to ensure that impurities are removed rather than just spread around!

Surface cleaning

There are three options:

• etching,

• cleaning with abrasive jet,

• disk grinding.

  ◦ Etching
  This involves chemically pickling the surface layer of aluminium so that it will accept the wash primer.
  The etching medium is a phos-phoric acid solution that is applied liberally to all the surfaces to be treated. It is applied with a brush, cloth or sometimes a mop, taking care to protect the operator from splashes.

  • Cleaning with abrasive jet
  Cleaning with the abrasive jet is carried out with an abrasive suitable for use on aluminium alloys, such as corundum or any other inert abrasive. This treat-merft must always be done on surfaces that are clean and dry, and be followed by thorough dust removal.
  Surfaces should be painted as soon as possible following abrasive treatment to prevent the oxide film from absorbing moisture or the treated surfaces from attracting impurities.
  Steel shot must not be used owing to the attendant risks of pitting corrosion. Abrasives that have already been used to treat metals other than aluminium should also be avoided for the same reason.

• • Disk grinding

• This type of preparation is used on surfaces that cannot be treated by etching or abrasive cleaning. It must be carried out with coarse grit wheels to achieve a well keyed adhesive substrate
  However, coatings do not adhere to surfaces treated in this way as well as after etching or abrasive cleaning.

• Painting

• Effective protection against an aggressive marine environment is obtained by multi- layer coatings in which each coat contributes to the efficiency of the system. The types of paint most widely used at present are based on polyurethane or epoxy resins.

• A typical system will consist of:

• •I a reactive primer, usually with a vinyl resin base and containing a certain amount of phosphoric acid.
  The primer ensures that the coatings adhere to the aluminium,
  • finish coats whose purpose is to reinforce the water tightness of the paint system and enhance its appearance.

• Finish coats should be applied with a compressed air spray gun.

• Fillers

• Special fillers are used to repair surface imperfections
  Preference should be given to sol-vent-free epoxy fillers as these are perfectly suited to immersion and will not shrink as they harden.
  Fillers should never be applied directly onto the metal but instead between successive coats of epoxy primer. Application is by spatula or more often using a float.
  Once it has dried, the filler should be sanded using wet or dry abrasive paper. Some fillers may require washing with fresh water after curing, especially if this occurs at low temperature. All dust should be carefully removed from the surface before the next coat is applied.

• Application conditions, paint casts, health and safety requirements

• As a general rule, paints should be applied:

• I in a dust-free, draught-free envi-ronment,

• I at temperatures above 15°C,

• I at between 40 and 70% humidity.

• Paint containers must be stored at approximately 20°C for at least 24 hours prior to use. Before commencing application it is advisable to ensure that the substrate is at least 3 °C above dew point.
  Painting and drying operations must be programmed as part of the overall production schedule in the same way as any other operation.