Beauty is only skin deep, but the quality of the end result depends entirely on how well the wood is prepared to begin with. When varnished, any imperfections (dents, scratches, etc) will stand out, especially if finished in a high gloss. Fill all holes or imperfections with a filler that is compatible with the top coating, especially if using two-pack polyurethane or epoxy resin; test on a sample if possible. Real men do not read instructions, but the manufacturer has spent a lot of time & money producing the product you have in your hands.
There are good reasons for handling their product in particular ways to gain the best result. The beauty of your boat & how long it lasts rests on how well you follow these instructions. Read them again even if you have used this product before – they may have improved it and changed a process. When varnishing new work, begin sanding with a coarse grade of sandpaper (120 grit) and work down toward the finest (320 – 400 grit). change paper as soon as it loses its grip & keep it clear.
Scraping will also provide an excellent surface. Dents in wood can often be removed by selectively wetting the indent with water (either drip it on or carefully apply it with a sponge). The wood absorbs the water & swells, the dent disappears needing only a light sanding. For the final finish, the finest paper (400 grit) well worn will give an excellent surface that is easy to varnish & looks great. When recoating existing varnish wash down with a good detergent & clean with fresh water. Then rub down with 320-400 grit paper if there are no nicks, blisters or imperfections. This removes any UV damaged varnish & roughens up the surface to give the new coating something to stick (key) to.
If there are minor blemishes, these should be repaired & sanded. Spot-coat the sanded area to level the surface, then sand before reocating the whole job. Generally two-pot varnishes will not successfully go over previous single-pot products. Check for compatibility & if there is any doubt, remove the old varnish completely before starting from the beginning. Varnish in poor condition needs to be removed by sanding, scraping or by paint & varnish stripper. Then treat as new timber & recoat. Mask off areas not being coated to avoid damage. Lay out drop sheets & make sure the area is clean & dust free. Check that the areas to be varnished are accessible; you cannot afford the time to move ladders etc. Remove all dust by vacuum cleaner & wipe the surface to be varnished with a rag slightly dampened with water to remove dust. Avoid touching it once clean. If working in the open make sure the surrounding area is dust free, dampening down the area to reduce any airborne dirt if there is even a hint of a breeze. Finally, just before applying the varnish, wipe again with a slightly dampened cloth to remove all traces of dust. Varnish only when the wood is dry as retained moisture in the timber can result in a milky finish to the surface.