Use a paint shaker for all coatings within 5 days of  application to ensure the material is thoroughly mixed. Use  a mechanical paint stirrer to mix larger quantities of material.  If a mechanical stirrer is driven by a drill, the drill should be  pneumatic, instead of electric. The sparks from an electric  drill can cause an explosion from the paint vapors.  The most important part of any painting project is the  preparation of the substrate surface. It takes the most work  and time, but with the surface properly prepared, the results  are a long-lasting, corrosion-free finish. Repainting an older  aircraft requires more preparation time than a new paint job  because of the additional steps required to strip the old paint,  and then clean the surface and crevices of paint remover.  Paint stripping is discussed in another section of this chapter.  It is recommended that all the following procedures be  performed using protective clothing, rubber gloves, and  goggles, in a well-ventilated area, at temperatures between  68 °F and 100 °F.  Aluminum surfaces are the most common on a typical  aircraft. The surface should be scrubbed with Scotch-Brite$^{®}$  pads using an alkaline aviation cleaner. The work area should  be kept wet and rinsed with clean water until the surface is  water break free. This means that there are no beads or breaks  in the water surface as it flows over the aluminum surface. The next step is to apply an acid etch solution to the surface.  Following manufacturers’ suggestions, this is applied like a  wash using a new sponge and covering a small area while  keeping it wet and allowing it to contact the surface for between  1 and 2 minutes. It is then rinsed with clean water without  allowing the solution to dry on the surface. Continue this  process until all the aluminum surfaces are washed and rinsed.  Extra care must be taken to thoroughly rinse this solution from  all the hidden areas that it may penetrate. It provides a source  for corrosion to form if not completely removed. When the surfaces are completely dry from the previous  process, the next step is to apply Alodine® or another type of  an aluminum conversion coating. This coating is also applied  like a wash, allowing the coating to contact the surface and  keeping it wet for 2 to 5 minutes without letting it dry. It  then must be thoroughly rinsed with clean water to remove  all chemical salts from the surface. Depending on the brand,  the conversion coating may color the aluminum a light gold  or green, but some brands are colorless. When the surface  is thoroughly dry, the primer should be applied as soon as  possible as recommended by the manufacturer. Before primer or paint is used for any type application, it must  be thoroughly mixed. This is done so that any pigment that  may have settled to the bottom of the container is brought  into suspension and distributed evenly throughout the paint.  Coatings now have shelf lives listed in their specification  sheets. If a previously opened container is found to have a  skin or film formed over the primer or paint, the film must  be completely removed before mixing. The material should  When purchasing the coatings for a project, always request  a manufacturer’s technical or material data and safety data  sheets, for each component used. Before starting to spray,  read the sheets. If the manufacturer’s recommendations are  not followed, a less than satisfactory finish or a hazard to  personal safety or the environment may result. It cannot  be emphasized enough to follow the manufacturer’s  recommendations. The finished result is well worth the effort. It is also highly recommended that compatible paints of the  same brand are used for the entire project. The complete  system (of a particular brand) from etching to primers and  reducers to the finish topcoat are formulated to work together.  Mixing brands is a risk that may ruin the entire project.  Purchase aircraft paint for the aviation painting project.  Paint manufacturers use different formulas for aircraft and  automobiles because of the environments they operate in. The  aviation coatings are formulated to have more flexibility and  chemical resistance than the automotive paint.  Topcoats must be applied over primers within the  recommended time window, or the primer may have to be  scuff sanded before the finish coat is applied. Always follow  the recommendations of the coating manufacturer.  Epoxy sanding primers have been developed that provide  an excellent base over composites and can be finish  sanded with 320 grit using a dual action orbital sander.  They are compatible with two-part epoxy primers and  polyurethane topcoats. Composite surfaces that need to be primed may include the  entire aircraft if it is constructed from those materials, or they  may only be components of the aircraft, such as fairings,  radomes, antennas, and the tips of the control surfaces.  The primer should be one that is compatible with the topcoat  finish. Two-part epoxy primers provide excellent corrosion  resistance and adhesion for most epoxy and urethane surfaces  and polyurethane topcoats. Zinc chromate should not be used  under polyurethane paints.  Mixing Equipment Preparation  Surfaces Primer and Paint 8-10