Question: All my walls and ceilings look great, made of Blueboard (plasterboard) skimcoated with plaster. I recall you writing that a primer was not needed when plaster is painted. Is this so? I am painting my rooms -- walls and ceilings -- and I want to do the best job possible. -- Christopher Davies, Bridgewater, Mass.Answer: It is so. As a rule, plastered surfaces do not need priming or applying an enamel undercoater. Two coats of a latex ceiling paint or latex wall paint are all that is needed. Be sure they are thin coats. The handyman feels strongly that this is so, because when his opinion on the subject is published or aired, he gets no brickbats or claims that he is wrong.
In the future, you can continue painting walls and ceilings, without priming.
If you really want to prime the walls, it will do no harm and may even do some good. If you do prime the walls, I suggest you use a latex enamel undercoater, also called primer-sealer.
For virtually every other surface -- bare wood or painted wood, painted metal, even bare metal -- you need a primer, also called a latex enamel undercoater, before finish painting. Undercoaters were developed to go over shiny surfaces without sanding, and work very well on all surfaces. They are particularly useful in preventing chipping of paint.
Question:I put up faced insulation (fiberglass faced with paper) with the paper showing in the room. A few wayward punctures from a back-pocket screwdriver created holes and tore the paper. Can I leave them when I put up plasterboard as a wall finish? -- Keith Buday, Holliston, Mass.
Answer: Sure, but it will make the paper backing (vapor barrier) useless. A vapor barrier with holes is no longer a vapor barrier. So, before putting up the wall finish, tape those holes and tears with duct tape or clear sealing tape of any kind; the clear stuff that makes a horrendous noise when you take it off the roll is best. If you have a lot of holes and tears, you can staple polyethylene plastic on the entire wall; while this is an extra vapor barrier, it will cover all those holes very well, and the wall finish will press it against the paper backing, creating, in effect, one vapor barrier.
The point of a vapor barrier is to stop the loss of air and heat from the house, and to do that, it must be in one piece. Yes, nails or screws used to hold the plasterboard do penetrate the vapor barrier, but it cannot be helped; engineers have not figured out a punctureless fastening system.
Question:The concrete slab floor of my new addition (12 by 16 feet) was poured 1/4 to 1/2 inch lower then the house floor. Is there a way to slope that floor so that it will look level? I can't put a threshold over the lip because the opening to the addition is as wide as the addition itself. --Mittie Jackson, Brockton, Mass.
Answer: Trying to slope the floor is impossible to do invisibly; you would see it for an eternity, and it would bug you. What bugs me is how the men who poured and smoothed the concrete could miss the existing floor by that much.
If there were a vapor barrier of plastic sheeting under the newly-laid concrete, then you could glue 1/4-inch plywood to the new floor, evening out the difference. This might not work because, as you say, the lip varies from 1/4 to 1/2 inch. If there is no vapor barrier under the slab, this cannot be done.
So, the only cure is to put a bonding agent on the new floor, then a skimcoat of mortar to even everything out.
Well, it is not quite the only cure; getting back to the threshold, you could do this, if you don't mind a threshold of sorts covering the entire opening to the addition. You could buy an oak plank 12 long and bevel the edges to make a threshold-type board. Glue this board on the high or low edge of the floor with an adhesive caulk. At least this is a sight easier and less expensive than pouring a new floor.
Question:The plaster in one of the rooms of my 100-plus-year-old house has a crack going from mantel to ceiling, and is bulging along the crack. I cannot push the bulge back into position; I think I tried. How can I fix it? -- Peggy Murdock, Milton, Mass.
Answer: If you cannot push the plaster back into position, you have to cut out the bulging plaster, exposing the wood lath underneath. Make sure the lath is still attached to the studs. Nail or screw on a plasterboard patch. You can't nail into the lath; it is too bouncy. You can nail into studs, but you must screw into lath, predrilling pilot holes.
Then fill the joints with joint compound, smooth off and apply paper tape, smooth joint compound over the tape, let dry, and sand smooth. Repeat the last three steps twice more. No kidding; anything less will show.
Globe Handyman on Call Peter Hotton will answer reader questions. Write him at the Boston Globe, Boston, MA 02107.