PREPARATION FOR VARNISH WORK
To achieve a good finish, the condition of the existing varnish should be thoroughly checked to determine the extent of the preparation required.
1. Look for areas of damage, separation or peeling or any other signs that the varnish does not have a firm hold on the substrate.
2. Clean and prepare the surface by washing with Special Thinner 216 to remove any contamination. Then inspect again to ensure no damage has been missed.
3. VARNISH - GOOD CONDITION - NO DAMAGE: Sand with 220-320 grit wet-or-dry sandpaper and when dry, wipe with a cloth dampened with 333 (2333N if varnishing with Perfection).
4. VARNISH - GOOD CONDITION - SOME DAMAGE: These areas may be repaired using Interstain Paste Wood Filler Stain. Spot priming and varnishing may then be required and the area sanded prior to the full varnish job.
5. VARNISH - POOR CONDITION: If the previous varnish coating is cracking, peeling or generally showing signs of separation over the whole area, it should be totally removed by either scraping, sanding or with a chemical paint stripper such as Interstrip 299E.
Tech Tip: Always work in the direction of the grain, whether sanding or applying varnish. This will avoid scratches that can even show through many coats of varnish.
Varnish Features, Hints & Tips
1. We recommend that the first coat of varnish applied is thinned up to 15%-20%. This will promote good penetration of the surface, and adhesion of subsequent coats.
2. After the first coat has been applied, the surface will appear rough. This is a result of the exposed ends of grain absorbing the varnish and lifting. Sand smooth with a 220 grit sandpaper.
3. Applying varnish with a brush is usually the best method, although roller application can be effective on large flat surfaces. Brush out the varnish with firm strokes along and across the grain holding the brush at 90° to the surface. Then "tip-off" by gently stroking the surface with the brush at 45°. Your brush should be used for varnish only.
4. A minimum of three full, unthinned coats is recommended for all varnishes. However, to achieve long lasting protection, you should plan to apply up to ten coats, depending on the system. As the number of coats increases, sand in between to increase the level of gloss and depth of luster.
Original article from Interlux (pdf)