TO PREPARE AN OLD WALL FOR PAINT OR PAPER.
First cut out all the cracks V shape, clean out the holes and bevel the edges same as the cracks. Then fill with fine plaster paris mixed with thin glue size. Fill with care; when dry, sandpaper the filling smooth and level. If the wall is sandy or rough, sandpaper it smooth as you can. If the holes are large, have a plasterer stop them, if you can; if you fail in that, and the job must be done soon, fit in thin boards, fill around the edges with plaster, and paste on cloth, or extra paper; but to do a nice job you must insist on having the large holes plastered. If the hole is up out of reach, and too large for you to fill, cement the edges with plaster, stretch a piece of cloth, or extra thickness of paper over it, and it will look all right, because the paper will shrink tight when it dries. If you find places where the clinches are broken, and the plaster is loose, press the plaster back to its place if you can, and cut small holes through the plaster and turn small broad headed screws into the lath even with the plaster and cement around the screws with plaster paris.
If it is a smooth wall with rough, sandy patches, sandpaper down the patches a little below the level of the wall, sweep out the loose plaster, give a coat of glue size, and knife or trowel in a coat of plaster paris mixed with glue size or vinegar, and when dry, sandpaper until smooth and level.
There are several points to be considered and provided for in filling cracks in a plastered wall preparatory to painting. First, are the edges of the cracked wall level? To determine this, lay your rule across the crack, and if you find the plaster on one side of the crack higher than the other, it shows that side of the wall has sprung out of place, because the laths are loose or the clinches are broken. The first thing on the program is to get the highest edges back to “place.” Failing in that, the next best thing is to raise the other side. If that scheme don’t work, the next method is to use sandpaper on a block and rub down the highest side with a wide bevel to match the lowest, otherwise your filling will be at an angle more or less acute with the general surface of the wall, and cast a shadow or reflect the light according to which way the light falls upon it, and the place where the crack was will “show” in spite of your best efforts to conceal it. If you find one edge of a crack higher than the other, gently press against it, and if it goes back to place, cement it with plaster paris wet up in clear water, and it will set in three minutes hard enough to hold the plaster in place. If the loose edge will not go back by gentle pressure, lay a piece of board over it and push hard as you dare to and not crush the plaster. If it is still obstinate, drill out a piece and insert a bent wire or other instrument made on purpose, and see if you can feel the obstruction and remove it. Failing in this, see if you can raise up the lower side to a level with the highest and cement it fast. If the last scheme is too much for your patience and ingenuity, resort to the block and sandpaper, and rub down the high side with a wide bevel to match the other. The next point is to prevent the paint near the edges of the crack, and on the filling which we put in, from drying flat while the balance of the wall bears out a gloss. To do this we must find out the cause of the “flatting” near the edges of the crack and over the “filling.” If we examine into the matter, we will find that when the wall cracked the plaster adjacent was more or less fractured and made more porous than the uninjured portions of it. Hence, more oil is drawn from the paint near the crack than where the wall is solid. Now, for the remedy: With a small pointed brush wet the edges of the crack with linseed oil until they will take no more in. Let the oil dry, and fill the crack with plaster mixed with thin glue size, but have the top of the filling one-sixth of an inch below the surface of the wall. Let the filling dry, and with a fine pointed brush paint over the top of the filling and the edges of the crack. Let the paint dry, and finish filling with hard putty. Let the putty dry, and sandpaper the job smooth and level. If you have to bevel the highest edge with sandpaper, first fill the beveled portion with oil. Let the oil dry, and fill the pores with hard putty, because the part beveled with sandpaper will be more porous than the balance of the wall. Treat and fill all small holes by the same method. Filling cracks in this way is a little tedious, I admit; but it is the only way that I know of to stop a crack in plaster, so it will stay stopped and not show after it is painted.