Wood grain fillers are used to fill pores in naturally porous wood, such as oak, ash mahogany and walnut. If you don't use a wood filler on these types of wood, the air trapped in the pores will leak into the finish and cause air bubbles to be trapped. Some wood grain fillers can obscure the wood grain, while others simply seal the pores without altering the wood's appearance.
Not all putty-type fillers can be used to fill wood grain -- most are designed to fill in dents, deep scratches and nail holes. However, some putty-type fillers can be watered down for use as a grain filler. This type of wood filler may obscure the wood grain as it fills in the pores, so it may be best to use if you intend to paint over the oak. Test the filler out on a small area or scrap of wood to see if it will work better with stain or paint. The floor should be stripped completely and sanded before using wood grain filler.
Clear Wood Grain Fillers
When you simply want to close up the oak grain's pores without obscuring the wood grain, use a clear, waterborne wood grain filler. This type dries clear so you can stain the wood, or apply polyurethane without staining if you prefer the natural look of the oak grain. Clear wood grain fillers may be applied with a brush or trowel. Although you may be left with a smooth surface once this filler dries, you still should sand lightly before applying stain or wood sealer.
Most oil-based wood grain filler comes with some sort of tint to help match the wood grain you are trying to fill. This can be helpful, but may change completely the color of the wood you are working with. Once dry, most oil-based fillers will not accept stain very well, so you may want to consider adding stain to the filler before you apply it. See the label on your wood filler to see if you can mix stain with it. Oil-based fillers may be applied with a brush or trowel.
Homemade Wood Filler
During the cutting and sanding processes, you will be able to collect a lot of dust from your oak woodworking project. Since many wood grain fillers are made of glue and wood, you can use this sawdust to make your own grain filler. This actually is preferable in some cases because your homemade filler will definitely match the color of your wood. Use three parts glue to one part sawdust. Use yellow glue to preserve the wood grain's appearance.
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