Check for paint build-up when repainting. In this photo, the old paint was properly removed, which allows a smooth undercoating application.

Many wood, metal, and fiberglass surfaces require a primer or an undercoater to smooth them out and provide protection against corrosion and water penetration, and to improve adhesion for finish coats. If these coatings are applied correctly, it can enhance the final paint job; incorrectly, it can ruin it.

Following are tips to solve problems that may occur with primer and undercoater use. Two major difficulties occur when the coating loses adhesion or when it cracks after application.

One possible reason for lack of adhesion is that the paint was applied to a wet surface. To remedy this, sand the peeling area thoroughly, allow it to dry if damp, seal off any source of water and then repaint it. On the wood surfaces, clear wood sealer should always be applied before repainting with an undercoater.

Peeling can also result from an improperly cleaned metal surface. If the metal was not cleaned correctly, prepare the contaminated surface as directed by the primer instructions and repaint the primer.

Another cause of peeling is too much paint build-up. The old paint has become "dead" and must be removed to the bare surface.

Another common problem is cracking of primer or undercoating. This typically occurs when the coating is applied to an incompatible surface or when wood surfaces have not been properly sealed. If the cracking is severe, the entire film will have to be removed. Before any undercoater is used again, it should be patch tested over any paint that is left to ensure that the undercoater and the remaining paint are compatible.

If the cracking is minor, the areas can be spot-glazed with a product such as Petit 7125 glazing compound and lightly sanded before a finish coat is applied.

Rust is the big concern for steel surfaces. Before repainting, remove all the rust to bright metal. On aluminum, the metal should be sanded to create a rough surface to which the primer can be bonded easily. Do not use steel wool, as it contains oil that will remain behind to ruin paint adhesion and tiny steel particles left behind on the surface can form rust patches.

Following these guidelines will help fix these problems before the finish coat is applied, saving time and frustration at the end.

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