The main purpose of a Grain Filler is to provide a completely flat surface for a finish to lay on. However it is also an excellent way to simultaneously add some contrast to the figure of the wood.
Several of the wood’s that are commonly used in guitar building are “open grain”. Mahogany, Ash and Limba are the most common. These woods have much larger pores on the surface of the wood than other species such as Maple or Alder.
The pores are large enough that when the wood is being finished, a consistent film will not form on the surface without small pin holes. Some builders deliberately skip the Grain Filling step as it can create an older, vintage look. This allows the grain pattern to translate further into the surface of the finish.
If you are after a traditional, mirror flat finish, and are finishing an open grain wood, Grain Filling should be a step in your finishing schedule.
Oil Based or Water Based:
Oil Based
Up until recently, Oil Based Grain Fillers were the common choice amongst finishers. Gibson famously used red tinted oil based fillers on the back of the original Bursts. It fills the grain and stains the surface of the wood at the same time. It’s a very practical method for bringing out the beauty of the figure in the wood while also prepping the surface. It is very easy to work with, having a consistency similar to peanut butter and it can be thinned with Mineral Spirits. Since it doesn’t dry as rapidly as water based fillers, you have a longer window for scraping and buffing the excess off.Water Based
Water based fillers have not been around for as long but have quickly found a strong user base. They dry very quickly which is beneficial if time is a factor in your finishing schedule. You can also clean up excess with a damp rag. They dry very hard and don’t present the risk of swelling the grain during lacquer application.
Natural or Tinted:
When selecting the correct filler for our instrument, you’ll want to choose a color that’s at least the same color tone of the wood, and in many cases, darker.
Natural is a common choice for Ash and White Limba. It’s a medium tan color that is often consistent with the current grain color.
Choosing a darker color will immediately accentuate the grain figure and provide a natural contrast. If you apply a tinted Oil Based Filler to bare wood, it will stain the wood as well.
If you don’t want the Grain Filler to color the wood outside of the pores, you can spray a washcoat prior to Grain Filling. A common method is to spray a full coat of 1 part lacquer (or sealer) to 1 part thinner. Let dry overnight, then proceed with the Grain Filling. This will seal only the flat areas of the surface, and leave open the pin hole sized pores to accept material.
