How to Paint an Aluminum Motor Block

by Timothy Burns
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The keys to every successful paint project are surface preparation and coating application. In the case of an aluminum engine block, the process is complicated by the presence of oily grime and the unique surface properties of aluminum. Backyard mechanics typically perform cleaning and painting of an aluminum engine block during the larger process of rebuilding an engine.

Project Preparation

Step 1

Remove and disassemble the vehicle's motor. As the engine block is disassembled, identify the individual parts that will be painted, and set them aside. Because of the labor-intensive nature of this process and the multiple steps involved, walk all the parts through this process simultaneously.

Step 2

Begin the cleaning procedure by having the bloc "hot tanked" by a reputable engine rebuilding facility. Dip the entire engine block in a tank of hot petroleum solvents which will soften and remove existing dirt and grime. While different processes are available for cleaning aluminum motor parts, many of them, such as sand and soda blasting, will negatively affect the aluminum motor surfaces. For this reason, hot tank cleaning is the recommended method for cleaning aluminum heads and engine blocks.

Step 3

Examine the parts once they are clean and cool to determine if any residual petroleum film remains on the aluminum surface. This residue interferes with coating adhesion during the painting process. If this film exists, clean the parts a second time. If no film is present, skip the next two steps.

Step 4

Protect all smooth metal surfaces that will receive gaskets during reassembly by covering them with masking tape and cardboard. The gasket surfaces must be smooth, free of paint debris and undamaged when the engine is reassembled.

Step 5

Bead blast any solvent residue from the engine block and related "hot tanked" pieces. The glass bead blasting material will polish the soft aluminum surface and prepare it to receive new coatings.

Surface Preparation and Priming

Step 1

Re-apply new masking tape to all engine block surfaces which will receive gaskets during reassembly. Some of the tape may have been damaged during the bead blasting process. Apply masking tape over the openings to any internal areas. Paint should not be applied to gasket mating services or to the engine block's internal areas.

Step 2

Paint bare metal surfaces with a commercial spray gun and quick-drying, high-temperature primer designed for adhesion to aluminum. Prime the entire exterior area. Follow all manufacturer's safety guidelines.

Step 3

Allow primer to dry thoroughly before applying topcoat paint.

Finish Paint Coat

Step 1

Select high-temperature rated finish coatings which are matched to the primer. Every paint type is produced to provide superior performance for specific conditions. Select paint products which match manufacturer's recommendations and the operating condition of the engine.

Step 2

Apply at least two coats of finish coating. A more satisfactory result will be obtained by applying multiple light coats of finish paint rather than one heavy application. Allow to dry thoroughly.

Step 3

Remove all masking tape and visually inspect all surfaces which were protected. Use paint solvent to remove any paint-over spray or coatings which were applied to gasket surfaces. Paint spray on gasket surfaces will cause the gaskets to fail during engine use.

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