Interior Surface Repaint Tips 

Interior surfaces are quite often re-coated with “same on same”, i.e. where the new coating system specified is a replacement of the same type of coating.

The type and the condition of the coating that currently exists on the surface to be repainted is determined by the specifier or painting contractor.

Where a change in the type of coating system is desired or required, special attention must be made to assure that the new system is not only appropriate for the use, but also is compatible.

Where a new coating is applied over a chemically different coating, various problems can arise. Many of these problems can be attributed to one of two incompatibilities;

Binders 

• Problem - the binders used in the two coatings are not compatible, which can lead to poor adhesion in the dry film. 
• Solution - a “tie coat", that is compatible with both the old and new coating is recommended

Solvents

• Problem - the previous coating is incompatible to the solvents in the second coating, which can cause lifting, bleeding, etc.
• Solution- a “barrier coat", which is compatible with both coatings, does not contain solvent to affect the original coating and is resistant to the solvents in the new coating, is used. 

Gloss considerations
Repainting must take into consideration the gloss as well as the age of the previous coating. A “same on same” application where the previous coating is glossy, must be prepared properly or a failure can result. A glossy surface does not allow penetration of the new coating, affecting adhesion and subsequently leading to peeling, flaking and cracking. Most coatings that cure by chemical reaction can continue to harden and become difficult for another coating to adhere to (even if the new coating is a similar and compatible product).

Re-coat Window
It is important to consider the minimum and maximum re-coat time (i.e. the re-coat window) for any coating system. With some coatings, this can be as short as one to two weeks, or as long as six months to a year. During the window, normal preparation can be used. After the window, even extraordinary preparation may not be enough and a tie coat may be necessary.

Surface preparation.
In both of these cases (i.e. glossy surface or outside 'the re-coat window'), surface preparation is critical. Abrading the surface creates an “anchor” pattern. This increases surface area and provides mechanical adhesion to the surface.

 Colour Change
As a general rule with a colour change, budget grade paint systems may not provide adequate hiding. Premium grade paint systems should be specified. Also, with semi-transparent stain, it is virtually impossible to change to a lighter colour without removing the entire previous system.

Two actions, which should be considered part of the normal procedure in ensuring a surface is suitable for repainting, are the appropriate testing of the surface to be repainted and the application of a test patch.

Comparisons of products 

• High-Performance Architectural Latex offers substantially better performance than Conventional Latex products in the areas of washability, burnish resistance, and ease of cleaning.
• The mar resistance of High-Performance Architectural Latex is better than W.B. Light Industrial Coating.
• W.B. Light Industrial Coating provides better chemical resistance than Conventional Latex or High-Performance Architectural Latex, and almost equal performance to Alkyd in some circumstances.
• Alkyds have less alkali resistance than Latex.
• Epoxy and Polyurethane coatings provide the highest level of chemical and abrasion resistance. They are much superior to both Latex and Alkyd coatings.

Preparing Bare Wood for Painting

 

There are hundreds of species of wood used to build a vast array of articles and structures for both exterior and interior exposures. Wood is also unique because it is the one substrate that can have an opaque pigmented finish; stained pigmented finish; or a clear finish.

Surface Preparation, General
Regardless of whether a pigmented, stained or clear finish is specified, the following are recommended practices for proper wood surface preparation.

Wood can be clear coated to highlight it's natural finish.

 Determine substrate
Before you carry out any cleaning it is best to determine what type of coating has been previously applied to the surface or substrate (if any)

Low gloss coatings -  are less durable than higher gloss coatings making them more susceptible to damage from harsh abrasion and chemical cleaning.

High gloss coatings - are more durable, so will endure more abrasion and chemical cleaning, but are less often used on interior walls because they tend to enhance any surface defects, making them more visible.

 Measure for moisture. Moisture readings should be taken, Exterior surfaces can not have more than 15% moisture present prior to painting; interior surfaces, 12%.

Remove surface contaminants. All grease, oil, dirt, dust, or other foreign matter must be removed from the surface. Avoid using steel wool to clean the surface: small fragments of steel wool can get caught in the grain and cause black spots to form long after the wood has been finished and installed. Rough surfaced woods can be cleaned of most loose material with a wire brush. A stiff fibre or hog’s bristle brush is recommended for smooth finished woods to reduce scratching.

Remove sap and pitch. Especially on softwoods such as pine and fir, all sap and pitch must be removed. All pitch pockets and sap-effected areas must be sealed with an appropriate sealer. If a stain or varnish/stain system is to be applied, the spot sealer must be applied after the stain and allowed to fully dry, otherwise, the colour may appear blotchy and inconsistent.

Deal with knots: If a paint system is specified, all tight knots should be sealed with an appropriate knot sealer. If a semi-transparent stain system is specified, knots should be left unsealed or the colour of the finish may appear patchy.Loose knots should be removed and filled with an appropriate caulking compound.

 Fill the holes: Holes, cracks, and joints are to be filled with an appropriate filling compound level with the surrounding surfaces. Openings in excess of 1/8"inch (3.18mm) should not be the responsibility of the painting contractor.

Sand with 150 grit sandpaper: For finished lumber, sand out any raised grain with 150 grit sandpaper before and between coats to maintain a uniform appearance. All sanding must be done in the direction of the grain, particularly when a stain or varnish system is specified. Sanding across the grain creates scratches that will absorb stain or varnish unevenly, a flaw that will need to be corrected prior to coating application; we’ll discuss this in more detail below. Dust particles from sanding should be removed by wiping the sanded surface with a tack rag, a loosely woven cloth dipped into a varnish oil and wrung out to make it tacky or sticky.