How to Check the Surface Before Painting to Avoid Visible Sanding Marks and Repair Outlines?

Short answer

The surface should be checked before painting, before base colour or effect paint is applied. If sanding marks, waves, pores, transitions or repair outlines are already visible in the filler primer, they will usually be even more visible in the final paint.

The best way to check the surface is to combine inspection against light, checking by touch and using guide coat powder or a guide coat spray. This highlights low spots, sanding marks, pores and poorly feathered repair edges that might otherwise appear only after painting.

Why is visual inspection alone not enough?

The surface may look ready at first glance. However, after paint or clear coat is applied, the surface reflects more light and surroundings. This makes uneven areas more visible, even if they were not obvious in matte filler primer or on a sanded surface.

This is especially important with glossy clear coat, metallic paints and effect paints. PMcolours effect paints such as Chameleon / Flip-Flop, Candy, Pearl, Ghost Pearl, Hologram or Glitter are sensitive to substrate quality, similarly to metallic paints.

What is guide coat powder or guide coat spray?

Guide coat powder or guide coat spray is used to reveal uneven areas while sanding filler primer. A thin contrasting layer is applied to the surface. During sanding, it shows where the surface is already level and where low spots, sanding marks or pores remain.

If guide coat remains in scratches, pores or along repair edges after sanding, the surface is not yet fully even. In that case, it is not suitable to continue directly to paint.

How does guide coat help?

it reveals deeper sanding marks,
it shows pores in filler primer or body filler,
it highlights low spots and waves,
it helps check surface flatness,
it shows poorly feathered repair edges,
it helps prevent defects from appearing only in the final paint.

Checking by touch is very important

Many uneven areas can be felt with your fingers more easily than they can be seen. The surface should therefore be checked not only visually, but also by touch. Fingers often reveal an edge, wave or low spot that is difficult to see on a matte surface.

Do not press only on one spot. Move smoothly across the repair with a larger area of your hand and from different directions. If you can feel the repair edge with your fingers, it will probably be visible after painting.

Checking against light

The surface should be checked from different angles and against light. Side light often reveals waves, transitions, filler outlines or uneven areas that are not visible under normal viewing.

On larger areas such as doors, bonnets, fenders or wheels, it is worth checking the surface slowly and step by step. The glossier the final clear coat will be, the more visible substrate defects can become.

Why use a sanding block?

When sanding by hand, it is important to use a suitable sanding block or pad. Sanding only with your fingers can create local low spots, waves or unevenly sanded areas.

Fingers apply uneven pressure to the abrasive. This can cause some areas to be sanded more and others less. On larger flat or slightly curved surfaces, a sanding block is usually better because it spreads pressure more evenly.

Sanding marks can appear even with the correct grit

Choosing the correct grit alone does not guarantee a good result. Scratches and uneven areas can appear even with the right abrasive if the sanding technique is wrong, pressure is too high or sanding is done in only one direction.

Even movement, moderate pressure and regular surface checking are important during sanding. If too much pressure is applied to the abrasive, deeper marks can be created and may become visible after paint is applied.

When should you not continue to paint?

Do not continue to base colour or effect paint if sanding marks, pores, patches, edges or repair outlines are still visible in the filler primer. Paint and clear coat will not hide these defects.

If a defect is visible at the filler primer stage, it should be repaired immediately. The surface needs to be sanded again, evened with filler primer if necessary, checked again with guide coat and only then continued.

How to prepare the surface for PMcolours effect paints

Our effect paints are among the more sensitive topcoats in terms of substrate preparation, similar to metallic colours. We generally recommend preparing the surface under effect paint with P800.

The surface must be even, clean, degreased, free from deep sanding marks and free from dust. The base colour before the effect layer is normally not sanded if it has been applied correctly and has no defects. The effect paint itself is also normally not sanded before clear coat.

What to do if a defect appears during checking

remove sanding marks with finer sanding,
even pores or low spots again with filler primer,
feather sharp repair edges smoothly,
treat sand-through bare metal with a suitable primer, ideally epoxy primer,
after repair, check the surface again with guide coat powder or spray,
do not continue to paint until the surface is level and even.

Surface checking process before painting

Let filler primer or the repaired layer cure correctly.
Apply guide coat powder or a light guide coat spray.
Sand with suitable grit and a suitable sanding block.
Check whether sanding marks, pores, low spots or sharp edges remain.
Check the surface by touch and against light.
Repair defects before applying colour.
After final sanding, clean, degrease and allow the surface to flash off.
Shortly before painting, an anti-static tack cloth can be used to remove fine dust.

Summary

The surface must be checked before paint is applied.
Guide coat powder or guide coat spray helps reveal sanding marks, pores, low spots and repair edges.
Anything visible or felt in filler primer will usually be even more visible in the final paint.
When hand sanding, use a sanding block rather than only your fingers.
Sanding marks can appear even with the correct grit if the sanding technique is wrong.
For PMcolours effect paints, we generally recommend P800 surface preparation.
Continue to paint only when the surface is level, even, clean and degreased.