When to use a filling primer

Filling primer is used to correct minor imperfections on a surface before it is painted with paint. However, it is important to know in which situations its use is actually necessary.

I use filler primer when a surface remains imperfect after filling or sanding. However, it is not suitable for every situation: on bare metal, on an existing sound surface or on plastics, other types of primer are often more appropriate. In line with the Carross method, this tutorial explains when to choose a filling primer, how to apply and sand it, and how it compares to epoxy primer and wet-on-wet primer. For an overview of the different types of primers, please refer to the [bodywork primer](/all-you-need-to-know-about-bodywork-primers) guide.

Key points at a glance. Filling primer is a 2K primer designed to reduce sanding marks and residual micro-defects. Once dry, it is sanded to achieve a perfectly uniform base before painting. It is mainly used after filling or on a surface that still has irregularities, when the flatness needs to be corrected. On bare metal, an epoxy primer is generally preferred; on a surface that is already sound and even, a wet-on-wet primer is often more suitable. This is the assessment that Carross technicians carry out systematically before choosing their primer.

1. Assess the situation first and foremost

• Filled surface, visible sanding marks — apply 2K putty in 2–3 coats, then sand smooth
• Slight undulations after rough sanding — use filler to fill the gaps, check under raking light
• Bare metal with no flatness defects — apply epoxy first, then filler only if a defect remains
• Sound, already smooth substrate — wet-on-wet application; using filler would be a waste of time
• Flexible substrate (bumper) — plastic primer followed by elasticised filler at the recommended ratio
• Small, localised touch up — spray filler to treat only the affected area

2. Equipment required

• Ultimate Blue abrasive discs — (1)
• Premium masking tape — (1)
• Premium masking paper (1)
• Primer + hardener (1)
• Primer spray gun (1)
• Filling primer spray cans (quantity 1)
• Epoxy primer / wet-on-wet (quantity 1)

3. Step-by-step repair guide

1. Prepare the surface. Sand, dust and use a degreaser on the area: filler will not adhere to a greasy surface.
2. Mask off. Protect neighbouring areas; the filler produces a heavy mist that is difficult to sand off elsewhere.
3. Adjust the spray gun. Choose a nozzle suitable for the filler’s viscosity and set the pressure according to the technical data sheet.
4. Apply the coats. Apply 2 to 3 even coats using a pyramid technique: start with a coat that covers the area generously, then gradually reduce the area covered with each subsequent coat. Observe the drying time between coats and avoid over-application, particularly on edges.
5. Leave to dry. The drying time for the filler must be completed before sanding: if sanded too soon, the filler will clog the sandpaper and still be too soft.
6. Sand flat. Rough-sand with a medium-grit sandpaper using sanding blocks, then refine the finish grit by grit up to the finishing grit, without ever skipping a grit, whilst checking the flatness under raking light.
7. Degrease. Remove dust and sanding debris before applying the paint.

4. Common mistakes to avoid

• Applying the filler in layers that are too thick: runs, incomplete drying through to the core, subsequent shrinkage
• Sanding before the primer is completely dry: the surface warps and scratches reappear
• Using the filler on bare metal without an epoxy primer: no anti-corrosion barrier
• Skipping a grit size when sanding: you go down too quickly and leave behind scratches you thought you’d removed
• Sanding without a dry Guide Coat: deep scratches may remain hidden in the filler and reappear under the paint.
• Neglecting the final sanding: unsanded filler retains the defects it was meant to correct
• Forgetting to remove dust between the filler and the paint: craters and grit marks

5. How to choose your products

The more pronouncedthe defect to be filled, the more justified the use of filler is compared to the ‘wet-on-wet’ method

Bare metalsubstrate → epoxy first; sound primer → wet-on-wet; uneven surface → filler

Spray gunformat for large areas, filler spray for touch ups

Paint system: stick to a rangeof primers compatible with the paint being used

Carross’s recommendation: Do not apply filler unnecessarily; select the type of primer based on the actual defects to be corrected.

6. FAQ

Is a plasticiser required in a filling primer?

On rigid substrates (sheet metal), no: the filler is applied as it is, in accordance with its technical data sheet. An elasticiser is useful when priming a flexible or semi-flexible substrate, such as a bumper, so that the film can accommodate any deformation without cracking. In this case, follow the manufacturer’s recommended mixing ratio.

Which nozzle should be used for a primer?

A filler is a thick, high-solids product: it requires a nozzle with a larger diameter than that used for paint, generally within the range specified on the product’s technical data sheet. A nozzle that is too fine will produce a fuzzy finish and become blocked, whilst one that is too wide will result in overspray. Always refer to the manufacturer’s recommendations rather than a universal value.

Finishing putty or filling primer?

They are complementary, not competing. Finishing putty fills defects still visible after structural putty has been applied, with high filling power. The filler comes next, to remove sanding marks from the putty and even out the entire surface before painting. Filler is used for its material properties; primer is used to achieve a smooth, even finish. At Carross, we recommend always applying both in sequence rather than skipping one or the other.

Filling primer or epoxy: which to choose?

They do not serve the same purpose. Epoxy protects bare metal and acts as an anti-corrosion primer; the filler fills in gaps and is sanded down to achieve a smooth finish. On repaired bare sheet metal, epoxy is often applied first, followed by the filler on top. The choice is therefore not mutually exclusive: it’s a question of the order of the process.

Tech’team’s advice

Apply the primer in a pyramid shape. Start with a coat that extends well beyond the repaired area, then gradually reduce the area covered with each subsequent coat. This method builds up thickness where it is needed whilst ensuring a smooth transition with the existing substrate. It also allows the solvents to evaporate properly. If applied in the wrong order or if uneven thicknesses are created, the solvents may become trapped in the lower layers. Sanding may then release these solvents and cause micro-bubbling beneath the paint. From the broadest to the finest: a better transition, more even drying and less risk of defects.

Explore our preparation / painting / finishing section and our bodywork guides and tutorials. Have a technical query? Contact the Carross Tech’Team: +33 (0)1 60 27 20 19.