Existing paint
Sound existing paint that needs cleaning and a uniform key.
Preparation & Spraying
How to Sand and Degrease a Surface Before Painting
Sanding creates the correct key for adhesion, while degreasing removes oils, wax, silicone, fingerprints and other contaminants that can spoil the finish. Follow the correct order to leave the surface uniformly dull, clean and ready for primer or paint.
A clean surface and a uniform key
Why sanding and degreasing matter
A surface can look clean while still carrying grease, wax, polish, silicone or fingerprints. These contaminants can cause craters, fisheyes, poor adhesion and peeling. Sanding creates a controlled scratch pattern that gives the next coating a dependable mechanical key and levels minor defects.
The order matters. Remove contamination before sanding so it is not driven into the scratch pattern, then remove all sanding dust and degrease again before primer or paint. The finished surface should be evenly dull, completely dry and free from visible residue.
Suitable substrates
This guide applies to
The correct abrasive and cleaner will vary with the substrate, its condition and the coating system being applied.
Bare steel
Steel requiring corrosion removal, refinement and prompt priming.
Aluminium and galvanised
Non-ferrous metals requiring controlled abrasion and compatible cleaner.
Plastic and uPVC
Suitable plastics prepared with mild abrasion and a compatible cleaner.
Wood and MDF
Porous surfaces sanded carefully and cleaned without being saturated.
Powder coat
Sound powder coating degreased and abraded to a uniformly dull finish.
Sanding and cleaning essentials
What you will need
Clean tools matter. Worn abrasives and contaminated cloths can leave scratches, residues and adhesion problems.
Abrasive paper and discs
A selection of suitable P-grades for defect removal and refinement.
Sanding sponges and pads
Flexible abrasives for curves, profiles, edges and difficult recesses.
Compatible degreaser
A panel wipe or cleaner approved for the substrate and coating system.
Lint-free cloths
Separate clean cloths for applying cleaner and wiping the surface dry.
Dust-removal materials
A clean vacuum, suitable tack cloth or clean, dry filtered air where appropriate.
PPE
Gloves, eye protection and suitable respiratory protection for the products used.
Step-by-step
Sanding and degreasing process
Clean first, sand methodically, then clean again. Do not apply primer or paint until the surface is uniformly keyed, dry and contamination-free.
Inspect and identify the surface
Confirm the substrate and whether it is bare, previously painted, powder coated, galvanised, plastic or porous. Check for rust, peeling paint, repairs, wax, silicone, grease and other contamination.
Read the primer or topcoat Technical Data Sheet before selecting the abrasive grade and cleaning method.
Remove dirt and heavy contamination
Remove mud, road film, salt, adhesive residue, polish and other heavy contamination before abrasive work begins. Wash or pre-clean using a method that is safe for the substrate, then allow the area to dry.
Never start sanding a greasy or silicone-contaminated surface because the abrasive can spread the contamination into the scratch pattern.
Degrease before sanding
Apply a compatible degreaser with one clean lint-free cloth, working on a manageable area. Before the cleaner evaporates, wipe the surface dry with a second clean cloth so the dissolved contamination is physically removed.
Change cloths frequently and never return a dirty cloth to the clean product.
Choose the correct abrasive
Use the least aggressive grade that will complete the task. Coarse abrasives remove rust, failed coating and heavy defects; finer abrasives refine the scratches and create the final key.
Use a sanding block on broad flat areas and a suitable sponge or scuff pad on curves, profiles and recesses.
Sand evenly and feather the edges
Use even pressure and overlapping strokes so the whole area receives a consistent key. Feather chips, repairs and paint edges until they feel smooth, with no hard ridge between the repair and the sound coating.
Take extra care around edges and raised details, where it is easy to sand through the coating.
Refine and inspect the scratch pattern
Progress through finer abrasive grades where required and remove the scratches left by the previous stage. Wipe away loose dust and inspect the surface in good, angled light.
The finish should be uniformly dull, with no glossy patches, missed recesses or deep scratches that the next coating cannot cover.
Remove dust and degrease again
Remove all sanding dust with a clean vacuum, suitable tack cloth or clean, dry filtered air where appropriate. Then repeat the two-cloth degreasing method using fresh cloths.
For porous or solvent-sensitive substrates, use only the cleaning method approved by the coating manufacturer and avoid saturating the surface.
Allow to dry and prevent recontamination
Allow the cleaner to evaporate fully and make sure seams, corners and porous areas are completely dry. Do not touch the prepared surface with bare hands or expose it to polish, silicone spray, oil or dust.
Apply the specified primer or paint within the recommended preparation window. If the surface is left exposed, inspect and clean it again before coating.
Abrasive selection
General abrasive grade guide
The objective is to remove defects and create a uniform scratch pattern without leaving marks that the next coating cannot cover. Start with the least aggressive grade that will do the job and refine coarse scratches before priming or painting.
P80-P120Heavy rust or failed-coating removal and initial filler shaping. This is normally too coarse as a final surface for primer or topcoat and must be refined.
P180-P240Refining repairs, feathering sound paint edges and preparing many repaired areas for a suitable high-build primer.
P320-P400Refining primer and keying many sound existing coatings before further primer or compatible topcoat, subject to the product instructions.
P500-P600Fine preparation before many colour coats where the Technical Data Sheet specifies this scratch profile.
P800-P1000Very fine wet or dry refinement and local defect correction only where the selected coating system calls for it.
Scuff padsUseful on contours, edges and recesses. Select the correct grade and make sure every glossy area is fully keyed.
Important: These grades are general guidance, not a universal specification. The substrate and the primer or topcoat Technical Data Sheet always take priority.
Useful answers
Frequently asked questions
Should I degrease before or after sanding?
Both. Degrease before sanding so oils, wax and silicone are not worked into the scratch pattern. After sanding, remove every trace of dust and degrease again with fresh lint-free cloths before applying primer or paint.
What grit should I use before aerosol paint?
There is no single grit for every coating. The correct P-grade depends on the substrate, the existing coating, the defects being removed and the primer or topcoat. Use the product Technical Data Sheet as the final specification and make sure any coarse scratches are refined before coating.
Can I use white spirit or a household cleaner to degrease?
Do not assume they are suitable. White spirit can leave an oily residue, while household products may contain fragrances, surfactants, waxes or silicones that interfere with paint. Use a cleaner specifically approved for the substrate and coating system unless the Technical Data Sheet states otherwise.
Should I wet sand or dry sand?
Both methods have valid uses, but the coating system determines which is appropriate. Wet sanding can reduce airborne dust, yet trapped water can affect seams, bare steel and porous materials such as MDF. Dry sanding is often easier to inspect and clean. Follow the product instructions and allow the surface to dry completely.
How do I know the surface is ready for primer or paint?
It should be uniformly dull, smooth enough for the selected coating, completely dry and free from dust, grease, cleaner residue and fingerprints. There should be no shiny missed areas, loose coating or deep scratches that the next coat cannot fill.
Ready for primer or paint?
A uniformly sanded, properly degreased surface gives the coating the best possible foundation. Check it in good light, keep it clean and follow the selected product instructions.