Natural but not rustic, silky smooth but not artificial, see but not feel the texture of the wood. This is what grain filler does for your guitar finish.
All wood is not created equal. Some woods have a more open grain than others, that is to say, irrespective of how far and how fine you sand, some types of wood will not sand perfectly flat and smooth for applying your finish and the texture of the wood grain will always be reflected in the finish you apply, unless you prepare and 'fill' prior to finishing. Ash is one of those woods.
Take a look at the pictures above, both guitar bodies have been sanded to 230 grit but it is clear that one has a vastly smoother surface than the other. This is the purpose of grain filling and the step that will turn a rustic looking guitar body into a spectacular looking finished piece. Many people assume that a glass smooth finish is achieved through tedious hours of sanding through ever finer grades, however this is not the case. Both in my furniture making and guitar building days I never sand higher than 350 grit, yet my natural finishes look and feel glass smooth (take a look at the picture below).
Lets consider for a moment what other advantages grain filling has, specifically colouring. Grain filler can be used to great effect to make the wood grain more visible if you are planning a coloured guitar finish. If you dye a grain filler dark and spray the guitar a light colour, subtle nuances in the grain that would otherwise have been lost in the colouring process are highlighted and thus retained and visible through the colour coat. However care should be taken when using this approach as dying the grain to a very dark colour can have an overly dramatic or pronounced effect which many people (including myself) find anaesthetic. For best results I'd recommend that you die the grain filler with a darker tint of the final colour you intend to use, thereby retaining the beauty of the grain pattern in a way that compliments rather than contrasts with the final colour.
There are also more subtle ways to colour grain filler in order to enhance the aesthetic of your finished guitar body. Take for example Olive Ash wood. This is a particularly beautiful wood that I use as much as possible for guitar building (the guitar below is olive ash). There is a clear line distinguishing the darker heartwood from the lighter wood (on the outer edges of the body) which in some cases contrasts a little too much and takes away from the shape of the guitar body. Where this occurs I use a natural or light grain filler for the darker heartwood and dye the grain filler a darker brown for use on the lighter areas of the body. Once this is sanded and finished the overall effect is to smoothen the contrast between the two wood tones and bring more balanced overall appearance to the body, while retaining all the beauty and natural look of the grain pattern. Its a subtle enhancement but when it comes to making the very best of a piece of wood, even with a natural finish (where options are limited), there are some choices you can make to enhance its final appearance.
Types of Grain filler
There are two main types of grain filler, oil based and water based. As a general rule of thumb if you use an oil based grain filler then the rest of your finish should be oil (solvent) based in order to have no issues with compatibility of top coats etc. I however use water based grain filler , irrespective of what type of finish is being applied. An oil based finish can be applied over a water based grain filler without issues so long as the grain filler has been allowed to completely dry. Water based grain fillers are quick and simple to dilute, easy to apply & sand, dry fast and clean up is a breeze. If you're new to grain filling, go water based. I use the type that comes in a tub like putty and thin it to apply.
How to apply grain filler
I will have a tutorial video soon (depending on the age of this post you may want to check now) but the quick version is:
Sand the guitar body and sides to 220 grit prior to grain filling
Two coats are always needed. There will always be some very small areas missed in the first coat that you will notice later.
A heaped teaspoon of grain filler should be sufficient for one side of a guitar. add one heaped teaspoon of filler and drop 9 or 10 drops of water into it and stir well. You'll be amazed how little water you need to thin the filler. You're looking for the texture of thick cream, soft cheese is too thick and milk is too thin (apologies for the example).
Work from one edge of the guitar to the other, pulling and pressing-in the filler, across the grain with an old credit card or similar. You should cover the entire guitar surface but not apply it so thick that you cannot clearly see the wood grain, so take off the excess with your last swipe. Don't overwork it, if you're applying it thin it will be too dry to manipulate after about 30 seconds so leave it and move on. You're doing 2 coats remember.
Wait 2 hours (assuming a normal household temperature)
Sand with 220 grit down to the surface of the wood but no further. I tend to do 20 seconds with an electric sander and when I'm almost there, finish with a sanding block. If its dusty when you sand its dry, if its tacky and sticking to the sandpaper its still wet, leave it another hour and retry.
Wipe with a microfiber cloth and vacuum with the brush attachment.
Carefully inspect for missed areas and repeat steps 3 & 4.
Leave overnight
Repeat step 6 but this time using 320 grit
Wipe and vacuum and inspect thoroughly for any missed spots.
All good? then you're all done and ready for finish. Just leave it a couple of days to completely dry first.
As with all things woodworking; getting each step right before moving on, makes everything that comes later, easier.