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What are we trying to achieve?

The general consensus is that, to achieve a mirror-like finish you must fill the pores on the wood before applying the finish. This assumes however that you are concerned to have a mirror-like finish. The high gloss of sprayed nitro-cellulose and polyurethane finishes, (both highly carcinogenic), seems to have set the standard. To be taken seriously by most buyers, instruments today seem to need to have this quality. It is NOT obligatory to fill the pores of any wood. To my mind, the more one puts on or into the wood, the less true its ability to vibrate.

Whether or not you fill the pores is a question of, where the pore filling is to be done, the possible effect on the sound, and what you want the instrument to finally look like. It is thought that a mirror finish lends a more elegant look to an instrument, whilst not filling them, produces a 'closer to nature' look. So, something about pore filling, (sometimes called 'filling the grain'..) and then one has a choice.......

Different Methods

Naturally all wood is different, and is used differently, according to these different characteristics. Pores can be small and hard to see, as for instance on cherry, poplar and maple. (Much used for the backs and sides of instruments). Equally, they can be large and clearly visible, as on ash, walnut and mahogany. (Much used as neck blocks). If the pores of 'open-grained' woods are not filled before the application of a finish, then they will still be visible afterwards. As with most areas, there are many ways to fill pores, and no concensus over which is best.

There are initially 2 basic choices if you wish to fill pores;

fill with finish;
fill with paste-wood filler.

The choice of pore filler is determined by the nature of the finish you wish to use. The method is to apply a number of coats of the finish, and sand back to wood. By doing this the finish will gradually fill the pores. When no more grain pitting can be seen, the pores are filled, and proper finishing can begin;

It is possible to do this with shellac, lacquer, spirit varnish and water-based varnish.
It is not possible with wax, oil, or oil/varnish blends. These finishes do not cure hard, so should not be built up into a thick film.

1. Pore-filling with Finish

Using a scraper to cut back the finish during the pore filling procedure is possible, but using sandpaper is safer.

Apply the finish
Use 220-320 grit sandpaper to cut back.
On flat surfaces, back with a cork or rubber block.
Use white spirit as a lubricant if wet and dry sanding. (water will raise the grain!)
Add a little mineral oil to slow down evaporation.
Increase fineness of paper to remove any scratches, up to about 600 grit.
Follow up with rubbing compounds (pumice or rottenstone- finer than 0000 steel wool)
Repeat until pores are filled.

2. Pore-filling with Paste

Paste wood fillers: either oil or water based, comprising a bulking agent, (often very finely ground silica) binder and carrier. The bulk fills the pores, the binder holds the material in the pores, whilst the carrier allows it to be applied. These can be bought coloured or neutral. If neutral, they can be coloured to match the wood as below..

	dry pigments	universal tinting colours	artists acrylics	Japan colours	artists oil colours
Water-based	Y	Y	Y	N	N
Oil-based	Y	Y	N	Y	Y