Finishing - Pore Filling Alternatives

Porous Woods need to be filled

Should one wish to achieve a high gloss finish on wood, it is recommended that the tiny pores visible in many types of wood are filled. Otherwise, a finish when applied on porous wood would seep into the pores and create an uneven surface. It would require several layers of finish, often laboriously applied, to eventually fill all the pores and create a flat even surface capable of manifesting a high gloss.

Filling wood prominent wood pores solely with a top coat could be problematic in some instances. Some types of finishes which are plasticky and highly reflective, create a glitter inside pores and could produce an unpleasant final effect. It is for these reasons that a pore filler is often recommended.

One alternative is a sanding sealer. These sealers are applied before the application of top coats and form a flat even base once sanded. Typically, these sanding sealers contain silica or stearates which settle in the pores and because of their low reflectivity do not interfere with subsequent top coats. They also sand easily and therefore produce a flat surface ready for the top coat. Most oil based polyurethane products have associated sanding sealers.

Also popular are the Nitro-Cellulose (NC) sanding sealers which are typically used with varnishes and lacquers and there is the popular Varathane sanding sealer which can be used with any type of top coat. One or two coats of such sanding sealers are generally enough to seal and fill pores in most woods. However, sanding sealers are not very effective when it comes to filling highly porous woods such as Sapele.

Dent fillers and various kinds of putties are also used to fill wood pores. These work best when wood is to be painted. Putties and dent can be applied with a jute hessian or a plastic spatula and forced into the grains. Once dried the surface can be sanded prior to applying a top coat.

Timbermate is perhaps the best marketed putty cum grain filler in the world. The makers of this product claim: "Timbermate does not shrink, sink, crack or fall out and has an indefinite shelf life. It is non-flammable, non-toxic and can be used to the last gram. The secret ingredient of Timbermate is tap water therefore containing no Acrylic, Latex or Solvents. Timbermate comes in 13 premixed colours plus Natural Tint Base. Timbermate takes all types of known stains, varnishes and coatings Mix 10% to 15% water into Timbermate to make a perfect Grain filler, Sanding Sealer and Prime Coat all in one application."

Timbermate is made up chiefly of Barium sulfate to which a small amount of quartz has been added. The binder is Guar gum and a little water is added to make the product into a paste like consistency. Barium sulfate is widely used for various purposes including in pigments, paints, adhesives and paper coatings.

Oak has large pores that often need to be filled

Other putties use similar ingredients or the cheap and freely available Calcium carbonate. Basically, any base material such as Calcium carbonate or Barium sulfate together with a binder (adhesive) and some solvent (water, glycol, oils and so on) can be used for making putties and dent fillers.

Putties and dent fillers, including ones like Timbermate, need to be tinted if they are to be used as wood fillers in wood that will be stained. Normally, the procedure is to first apply a thin coat of sealer (such as Shellac), followed by the stain and then the grain or pore filler. After the pore filler has dried, the surface should be sanded and ideally sealed before proceeding with the top coat. The grain filler should ideally match the stain unless a contrasting effect is specially desired.

In the West, several oil and water based products have been specially designed to act as pore or grain fillers. These are usually in dark colours and are of a thick consistency. There are, however, several other perfectly good traditional methods that will fill wood for staining or painting

Chalk Powder

Calcite or Calcium Carbonate Powder is a naturally occurring substance that has been traditionally used in India for a vast number of purposes including grain filling. This is readily available in local paint shops as Chalk power or Chalk Mitti. This is usually applied after the surface has been sanded and stained. The powder is mixed with a little water and sometimes a little Shellac and rubbed into the pores, a little at a time. The surface is lightly sanded after the powder dries. As layers of Shellac are bodied on, the powder becomes translucent and takes on the colour of the stain or wood under it. This makes chalk powder a cheap and excellent pore filler.  In painting applications, chalk powder is mixed with a little amount of water and paint and applied as a thick creamy paste which is then wiped cleaned and sanded after it is dried.

Calcite or Calcium Carbonate crystals: this is a cheap widely available substance

This is a box lid I was working on. Knot holes were filled with epoxy and then the pores filled with chalk mitti (French Chalk in Calcutta).

After pore filling, I polished the lid with Shellac. The result was a highly polished surface with no pores visible. The finished box below:

Plaster of Paris

Plaster of Paris is hemihydrated calcium sulfate and when mixed with water it undergoes a chemical reaction creating a hard plaster. This makes for a cheap and superb grain filler. This was a traditional method for pore filling and remains perfectly acceptable. However, most woodworkers have abandoned it for other better marketed products. In times past, even piano makers used plaster of Paris to achieve the high gloss finish associated with pianos. Here is a link to a superb article on the subject: http://pianomaker.co.uk/technical/filling/

Pianos and other musical instruments are traditonally given a high gloss finish

I have tried all these methods for pore filling and find the traditional ones the least complicated. In staining projects, I generally use a Shellac wash followed by the stain; after that dries, I either add a little chalk powder mixture or go with pumice as a filler. I have also used Plaster of Paris for a couple of projects stained and finished with Polyurethane. The resulting high gloss has persisted over five years without any problem.

Generally, though, Plaster of Paris is messy and dries very hard which makes it difficult to sand. For a Polyurethane top coat, I sand the chalk powdered surface and apply a thin coat of Shellac that helps adhesion of the subsequent coats. A very high gloss surface can be obtained by this method no matter whether the finish is of Shellac or Polyurethane.

Indranil Banerjie
9 June 2018