Painting over spackle or joint compound without properly sanding or priming it will result in an irregular surface or dull spots where the paint isn't blended in properly. Using the wrong type of primer may even cause shiny spots in the finished paint. The same goes for exterior spackle.
When to Use Spackle
Spackle (spackling paste) is one of several types of filler. Others include joint or drywall compound, wood putty, stucco and masonry patch and plaster of Paris. Spackle comes in a variety of forms. Lightweight spackling is commonly used for interiors. It dries quickly, doesn't shrink and sands easily. Stress cracks or joints that expand and contract are best filled with flexible elastomeric spackle. Vinyl spackle can be used on exterior siding and trim.
All spackle is intended only for smaller repairs and narrow cracks. Using the right spackle will ensure a long-lasting repair.
Painting Over Interior Spackle
If the spackle repair is on a smooth surface, it needs to be sanded very flush so there are no ridges. Feel for roughness by running your hand over the repair. If the spackle patch is on a textured wall, you may not have to sand at all.
Because a spackle patch is more porous than the surrounding surface, it needs to be primed before applying the finish coat, otherwise it will show as a dull spot. There is no need to use a special primer; any water-based general purpose or drywall primer will do. Use whatever paint you are using to finish the paint job. Allow it to dry for about four hours before recoating.
Avoid using a stain-killing primer on the patch. These are very dense primers and may make the primed spot less absorbent than the surrounding wall. Rather than resulting in a dull spot, you may end up with a shiny spot.
To achieve a blended, seamless finish for a touch-up, use the same application method that was used for the original surface. In other words, use a roller for a rolled wall, or a brush for a door that's been brushed. Extend the painting touch-up well beyond the edges of the patched area, and lightly feather the edges so it blends seamlessly.
Painting Over Exterior Spackle
Because exteriors are subject to direct sunlight and more temperature and humidity extremes, materials used for patching and painting the outside of your home have to be more flexible and resistant to the elements. A common mistake people make when using spackle outside is neglecting to prime bare wood before spackling. Bare wood is absorbent and will leach moisture from the patch, causing it to dry out and crack. Prime the wood, let it dry and then apply the spackle.
To paint over exterior spackle, use two coats of the finish paint, allowing four hours drying time between coats.