How to Check Effect Paint for Defects with Raking Light

Short answer: Raking light inspection is a method of shining a light along the wall, almost parallel to the surface, before and after painting, to make dents, bumps, filler edges and sanding marks visible. Raking light inspection matters especially with effect paint, because glossy, pearlescent and metallic effects reflect light and, in doing so, magnify small defects in the substrate. The check is done at least three times: after filling, before priming and after the final coat.

What is raking light inspection?

Raking light inspection is a way of checking how even a wall is by placing the light source at the edge of the wall and directing the beam so that it skims across the surface. Light coming from the front flattens and hides the small hollows and ridges on a surface; light coming from the side casts a small shadow behind every bump. These shadows reveal defects that cannot be seen under normal room lighting.

Painters and applicators also call this check "side lighting", "grazing light" or simply "holding a light to the wall". The method lets you spot in advance the defects that the sun, slanting in through a window during the day, will show on the wall. If a defect is noticed once the paint is finished, fixing it often means going back through coat after coat; if it is noticed at the filling stage, a small touch-up is all it takes.

Why is raking light inspection more important with effect paint?

Because effect paints create their look by reflecting light or by producing differences in sheen across the surface, every irregularity in the substrate shows up more clearly within the effect. A matt emulsion paint hides small defects to some extent; a polished, pearlescent or metallic surface, by contrast, highlights the defect along with the light.

The product pages on silkcoat.com also require an even substrate. The Marmarito page states that the surface must be "clean, dry and sound", and asks for Silkcoat interior acrylic putty to be applied first if the surface is not smooth. The Metalico page likewise says that if the surface is not flat, "a smooth surface must be obtained" with the same putty. Raking light inspection is the most practical way to see whether this "smooth surface" requirement has really been met.

Effect type	Silkcoat example	What stands out under raking light
Polished marble look	Marmarito	On a trowel-polished surface, bumps and dents appear as breaks in the sheen
Metallic effect	Metalico	Because the metallic pigment reflects light, sanding marks and filler edges become noticeable
Pearlescent and pearl-look effect	Pearl	Sheen that shifts with the angle of the light reveals waviness in the substrate

What do you need for raking light inspection?

For raking light inspection, all you need is a lamp you can aim, a room you can darken and a pencil for marking. These items make the check easier:

• Adjustable lamp: A handheld torch, a rechargeable work light or a floodlight on a stand. What matters is that the light comes from a single direction at a narrow angle.
• Dark room: Close the curtains and switch off the ceiling lights; otherwise light from the front wipes out the shadows.
• Pencil or coloured tape: Lightly mark the position of each defect you find on the wall.
• Repair materials: Keep filler, a filling knife and sandpaper to hand so you can make repairs as soon as defects are found.

How do you carry out raking light inspection step by step?

Raking light inspection consists of darkening the room, holding the light at the edge of the wall parallel to the surface, scanning the wall section by section and marking the defects. The sequence is as follows:

1. Clean the surface: According to the instructions on silkcoat.com, the surface must be free of all dust, dirt and grease. Dust particles can look like defects under raking light.
2. Darken the room: Close the curtains, switch off the ceiling lights and use only the inspection lamp.
3. Place the light at the edge of the wall: Hold the lamp at one end of the wall, very close to the surface, so that the light glides along the wall.
4. Scan the wall section by section: Move the lamp slowly from top to bottom and from left to right. Every shadow signals a bump or a dent.
5. Shine the light from the other side too: A defect that is invisible from one direction shows up from the opposite one. Repeat the check from both the left and the right, and from above if necessary.
6. Mark and repair the defects: Fill the marked spots with filler, sand them once dry and check the same spot again with raking light.

Which defects does raking light reveal?

Raking light reveals dents, bumps, filler edges, sanding scratches, trowel and knife marks, and dust particles stuck to the surface. Each defect looks different under the light and is fixed in a different way:

Defect	Appearance under raking light	Repair
Dent or hollow	Bright on the light side, shadow on the far edge	Fill with filler, sand once dry
Bump	A distinct shadow behind it, away from the light	Sand down, add a thin skim of filler if needed
Filler edge	A sharp line between the flat surface and the filled area	Feather the edges into the surface with sandpaper
Sanding scratch	Fine, parallel lines	Smooth out with finer sandpaper
Trowel or knife mark	Long, wavy bands of shadow	Sanding and a thin coat of filler
Dust particle	Small, dot-like shadows	Remove the dust from the surface

Once substrate preparation is complete, the instructions on silkcoat.com call for a primer depending on the product. The Marmarito page recommends Silkcoat Putty Primer for filled surfaces and Silkcoat Universal Primer for smooth and previously painted surfaces. For filler, the product pages name Silkcoat interior acrylic putty.

At which stages is raking light inspection done?

Raking light inspection is done at least at three stages: after filling and sanding, before priming, between coats and after the final coat. Any defect found at a stage is fixed before moving on to the next one:

Stage	What is checked	Related instruction on silkcoat.com
After filling and sanding	Dents, bumps, filler edges	If the surface is not flat, a smooth surface must be obtained with Silkcoat interior acrylic putty
Before priming	Dust, sanding scratches	The surface must be free of dust, dirt and grease
Between coats	Differences in coat thickness, tool marks	Allow at least 4 hours between coats for Marmarito and at least 2 hours for Metalico
After the final coat	Overall appearance and consistency of the effect	Full drying takes 24 hours for Marmarito and Metalico

Ambient conditions should also be watched during the check. According to silkcoat.com, the room and surface temperature must be between +5°C and +35°C while Marmarito and Metalico are applied and while the product dries. We explain how the marble look is achieved in our article on how to apply marble effect paint, and how to choose a metallic effect in our Metalico Soft guide. For the meaning of the terms, see our effect paint glossary.

Conclusion

• Raking light inspection is a method of holding a light to the side of the wall, parallel to the surface, to reveal defects that cannot be seen under normal lighting.
• Because effect paints reflect light, they magnify defects in the substrate; the product pages on silkcoat.com also require a clean, dry, sound and smooth surface before effect paint is applied.
• The check is repeated after filling, before priming, between coats and after the final coat; every defect is fixed before moving on to the next stage.
• For the right substrate preparation and product choice, get in touch with us; you can browse all products on the effect paints page.