Surface Preparation Guide – Everything you need to know to prevent coating system failures

Surface Preparation

Correct surface preparation will help prevent coating systems failures. The most common coating system failures caused by incorrect surface preparation are:

  1. Loss of adhesion (delamination) between the coating system and the substrate.
  2. Loss of adhesion (delamination) between the various coating layers of the coating system.
  3. Cissing, cracking or frying of the coating system.
  4. Gritty or cloudy finish in the final coat of the coating system.

Coating systems are not designed to hide or correct imperfections on the surface of a substrate, only correct surface preparation can rectify these (either on the bare timber substrate or between the coats of a coating system). All surface imperfections will telegraph through the coating system and be seen in the final finish unless corrected.

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Cleanliness

Coating application should always be carried out in the cleanest environment possible. While the need for a dust free, clean and a properly functioning spray-booth is easily understood, the need for personal cleanliness is most often overlooked. Many coating systems have been spoiled by applicators for example not washing their hands after eating a greasy lunch, or by wearing contaminated, dust coated clothing during surface preparation and coatings system activities. Maintaining a high level of cleanliness results in a consistently higher quality in coating system finishes.

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Surface Preparation - Prior to Coating

Use only correctly seasoned clear grade timbers from reputable suppliers. Beware of resin  bleeds and wood knots as these can cause problems in the coating system. Allow MDF board to stabilise prior to machining. Prepare the surface using the following procedure:

  1. Fill all defects with a wood filler (e.g. cracks, holes, etc.)
  2. Sand the surface to a smooth even finish. While the sanding process may start with 100 or 120 grit papers to correct surface defects, always finish:

(a) On Timber with 180 grit

(b) On MDF with 240-320 grit

  1. Remove all sanding dust using air guns and tack rags.
  2. Ensure substrate is free from dust, grease, dirt and all other contaminates.
  3. Ensure timber is stabilised to ambient conditions and has a moisture content of less than 15% immediately before commencing coating operations.
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Sanding

  1. Form panel edges and face profiles using sharp router blades at the correct spindle speeds to reduce wood fibre tear and machine chatter marks.
  2. Radius off exterior and internal corners to reduce coating pull back due to surface tension of the coating relative to the substrate.
  3. Take particular care on edges or profiles to ensure the total surface area has been sanded thoroughly. Most coating system failures occur on the edges of flat panels and are usually caused by insufficient surface preparation in these areas.
  4. On MDF Board edges use the correct grade of abrasive paper to minimise fibre tear.
  5. Inspect all edges after sanding to ensure the best possible sanded finish has been achieved.
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General Sanding Tips

  1. Sand with the grain of the timber using sanding blocks or mechanical sanders to maintain maximum flat area contact between the abrasive paper and the surface. Hand sanding without a sanding block will result in an uneven and poorly finished surface.
  2. Use only new abrasive papers of the correct grit size. Change frequently to maintain the correct cutting performance. Worn abrasive paper will polish the surface, preventing the sealer or pigmented undercoat from penetrating, and consequently, adhering to the surface.
  3. Use sanding equipment with dust extraction facilities to prevent powder build up under the abrasive paper. Excess powder will clog the abrasive paper and polish the surface, preventing the sealer or pigmented undercoat from penetrating, and consequently, adhering to the surface.
  4. Remove all surface dust using air guns and tack rags. The use of air guns alone will not remove all of the surface dust. Residual dust will cause a cloudy finish or gritty appearance in the topcoats.
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Surface Preparation - During Coating Applications

On the prepared  surface apply (as per the instructions of the relevant product data sheets) either a clear sealer or pigmented undercoat as required. Inter-coat surface preparation while similar to surface preparation of bare timber, does have peculiarities to be aware of.

All coating systems rely on two methods to adhere the next coat of the coating system onto the previous one.

  1. Chemical Bond: Where the previous coat is softened or dissolved a little by the solvents in the next coat allowing the resins of the two coats to bond together.
  2. Mechanical Key: Where the sanding marks left in the surface form a profile in the previous coat for the subsequent coat to flow over and shrink onto, through the drying and curing of the coating.

Both functions require that the previous coat be thoroughly sanded prior to the application of the subsequent coat. Some coatings can be applied wet on wet in double pass application – but only if indicated on the relevant product data sheet and within the time limits imposed.

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Inter-Coat Sanding Tips

  1. Allow the previous coat to fully dry (dry times are specified on product data sheets) before sanding – uncured coatings will ball up on the abrasive paper and scour the coating.
  2. Sand coating using sanding blocks or mechanical sanders to maintain maximum flat area contact between the abrasive paper and the surface – especially important at the junction where the panel flat faces meet the edges or profiles. Hand sand the contours of the edge and profiles, being careful not to remove the coating from the important edge and face junction.
  3. Take particular care on edges and profiles to ensure the total surface area has been sanded sufficiently.
  4. Use only new abrasive papers of the correct grit size as indicated on the relevant product data sheets – usually a finer grit than the abrasive paper used on the substrate surface. Change abrasive paper frequently to maintain the correct cutting performance. Worn abrasive paper will polish the previous coat and inhibit the subsequent coat from creating a good mechanical key.
  5. Sand immediately prior to beginning recoating operations. The chemical bond will be greatly reduced after sanding the previous coat especially if recoating is left too long (e.g. overnight or over the weekend) – this is especially important for two-pack products.
  6. Thoroughly sand a fully cured, older two-pack coating with 280 or 320 grit paper prior to application of a new coating system. This new coating system will have very little chemical bonding with the fully cured older coating and is almost totally reliant upon a mechanical key for inter-coat adhesion.
  7. Remove all surface dust using air guns and tack rags. the use of air guns alone will not remove all of the surface dust present. Residual dust will cause a cloudy finish or gritty appearance in topcoats.
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Problem Solving Guide

Already have a problem and need to know what you can do to fix it? Try using our Problem Solving Guide

For more information about sanding preparation contact Mirotone’s Technical Department.

Article provided by NZ Mirotone Branch.

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