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How to determine what type of primer paint you need

A building contractor wouldn’t think of building a house without first laying a solid foundation.

The situation isn’t much different when repainting a classic tractor, where the foundation comes in the form of one or more layers of primer. The problem is deciding what type of primer you need, especially when you consider the number of primers available.

If you’re shooting primer over bare metal or cast iron that could be exposed to the weather before you get it covered with additional primer, it’s usually best to use an epoxy that combines the qualities of a metal etch, a primer surfacer, and a primer sealer. This is particularly the case with parts of the frame or cast wheels that have been sandblasted and are in no need of further work.

Self-etching primers have to have another primer over the top of them or the paint won’t adhere. It can’t be an epoxy primer, but it can be a urethane primer. In the meantime, it’s important to talk to your paint dealer about the time window allowed between the primer and paint topcoat. Some are a few hours; others need several days.

Urethane primers

Although urethane primers are very popular due to their hard finish, they do not include any kind of chemical agent to prevent rusting. Consequently, you either need to ensure that the surface is completely free of rust before you apply a urethane primer, or you need to lay down a coat of epoxy primer or an etching primer and put the urethane over it. Otherwise, you may find rust popping through the surface down the road.

Since urethane is only a primer surfacer, you’ll also need to apply a sealer before you paint. Again, a coat of epoxy primer over the top will serve the purpose.

Filler primer paints are basically formulated to fill in any pits in the surface from sanding or rust even quicker than an epoxy primer alone. Some restorers believe filler primer is little more than liquid body filler. In most cases, the filler primer or high-build primer is also a urethane product. 

You can generally add as many coats of filler primer as you want, sanding between each coat (as some restorers do) or after every three or four coats. Because it has the texture to fill pits and imperfections in the sheet metal, an unsanded primer coat would be too rough to paint over without further preparation, which includes a sealing primer.

Sealing Primers

The final step before applying a coat of paint should involve applying a coat of sealing primer, or sealer. This closes the surface and prepares it to accept a coat of paint. Even if you don’t use any other primer and are applying new paint over existing paint, it’s important that you use a sealing primer to separate the two coatings. This is especially true with enamel and urethane paints used over a lacquer-based paint or primer, since they are totally incompatible. 

If there’s any doubt about what kind of primer to use at any stage of the painting process, it’s best to discuss it with your paint supplier. Or refer to the application guides or information sheets that are available for the products you plan to use. Like everything else, paint continues to evolve with new formulations and chemistries.

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