Rubbing off the filler in such a manner as to leave well filled pores and a level surface is the real test of a wood-finisher's skill. After painting on the filler the condition of the surface must be watched, and the excess of the dull mixture should be rubbed off crosswise of the grain at the proper time. If you rub the filler too soon, before the thinner has evaporated, the thickening body mixture of silex and pigment will pull out of the pores.
This is because it has not had time to stiffen and form firm anchors, and a poorly filled uneven surface will be the result, which can look terrible on wood for ornamental pediment, designer window toppers, or fireplace designs. If you wait too long, the filler will not come off the surface easily; and some of it may pull out of the pores, as it is too stiff, instead of being cut off just at the surface. The filler that has settled down into the ruptured cell cavities should not be disturbed, while all excess which is above the surface of the wood must be rubbed or scraped off.
Filler should be rubbed soon after it has dulled and no longer looks wet, for prompt action at that time is necessary to secure good results. You should not fill too large an area before beginning the rubbing process. Various materials can be used for rubbing off the filler from wood surfaces. The newly filled floors of buildings are frequently rubbed with soft shavings, saw dust, or excelsior, because these substitutes cost little or nothing and are available.
Shops and furniture factories generally use better material, such as hairy sea-moss or sea-grass which is commonly used in upholstering, jute or flax, or hemp tow, rags, burlap, or even cotton-waste which is objectionable if it leaves lint on the surface. Almost anything will serve the purpose if it is not too stiff and is somewhat absorbent, that will scrape or rub off the excess filler from the surface without digging it out of the pores. Crosswise rubbing of paste filler is the proper method, since rubbing lengthwise of the grain tears the filler out of the pores to a greater extent.
You should use a handy or pad of jute tow, sea-grass, or rags, and rub one surface after another crosswise with reasonable pressure, taking more material as soon as that it becomes too wet or full of filler in your hand. You can get a cleaner surface on frieze boards, wood frames , and plinth blocks by making a few light strokes with cotton cloth or rags lengthwise of the grain in finishing the process, so removing the cross-lines of filler left by the first hard rubbing .
Two or more coats of filler are sometimes required to fill the pores completely. If the first coat is too thin, a second application is added. Since upright or slanting surfaces do not hold the filler in the pores as well as horizontal surfaces, the mixture which is to be used on vertical surfaces should be quite thick, or it may run out of the pores. Frequent examination will show whether the pores are full of the filling material. A second coat can best be applied as soon as the first one has become set or dull.
After a filler has been rubbed, if you are inexperienced, it is wise to examine portions of various surfaces of a filled article using a pocket magnifier of from 12 to 18 enlargements, especially on wood for expensive pieces such as ornamental pediment, bar rail molding , or fireplace designs. A hand-glass of even lower magnification will help. If the filling is not well done, the glass will show it instantly. Additional coats of filler can be applied, if the pores are not full, after rubbing off the excess from the first application.
Many expert wood finishers think that it is best to paint another coat of rather thin filler over the rubbed surface as soon as possible, at least before the mass in the pores is thoroughly dry and hard. This second coat of filler can be rubbed when it has set just as was the previous application; and, typically, the result is a satisfactory level surface properly filled. Sometimes a second or third coat is given to a filled surface that has discharged hard, and the result is generally satisfactory.