R.B., Weymouth

A. Any hot zinc-dipped galvanized nail will do, 2 1/2 inches long. You can use finish nails, countersink them and fill the holes, but they don't hold very well on outside work. Better are casing nails but I have not seen them in my travels. Casing nails have tapered heads larger than a finish nail, but can still be countersunk and filled. You could use box nails, which have large heads but can be countersunk with some effort. Fill the holes and you're all ready to paint or stain.

Q, My screened and closed-in porch has a solid roof, with matchstick wainscoting as a ceiling. The wainscoting is nailed on top of the rafters, acting as roof sheathing as well as inside ceiling, exposing the rafters. Roll roofing was installed with extra-long nails, some of which show through the wainscoting. What, if anything, can I do about those nails?

FRUSTRATED CALLER

A. You can't pull the nails or cut them; even substituting short nails is not practical; it would mean ripping off the old roll roofing and relaying new. But If you want to keep the wainscoting showing, there is no other choice.

Here is one possible solution, but it covers the wainscoting. Cut Homasote sheets to fit between the rafters, prepaint with two coats of latex solid stain or latex ceiling paint and press into place, impaling the material on the nails coming through. You could nail a quarter-round molding along the edge to hold the Homasote in place and make a neat edge. Homasote is a heavy-duty papier mache material thqt comes in sheets half an inch thick.

The only other way to cover the nails is to nail more wainscoting to the rafters. This would cover the beams, of course, but it would keep the wainscoting look.

Q. Over the past year, I have seen gray marks on the nails holding the plasterboard wall onto the studs. I bought the house four or five years ago and fixed it up, and did everything I thought I was supposed to do. No one can tell me what causes those gray marks. Can you?

J.D., Cummaquid

A. You kidding? Well, I can certainly try to tell you, with an informed opinion. Moisture is the cause and it is no one's fault, really. What happens is that moisture (water vapor) builds up in the house, created by breathing, washing, bathing and cooking, and hits those nail heads, which are colder than the rest of the wall, even though they are covered with plaster. When the water vapor hits those cold spots, it condenses into water; you might not be able to see the water, but it's there. Then mildew will grow on that wet spot, or dust or dirt will land on it and stick, both creating those gray spots. That dust or dirt is no reflection on your housekeeping.

To determine what it is -- mildew or dirt -- touch a spot with a little liquid chlorine bleach; if it disappears in seconds, it is mildew, which can be removed and killed by wiping with a solution of one part bleach and three parts water. If the bleach has no effect, it is dirt, which can be washed off with any cleaner.

To try to prevent this from recurring, ventilate the house twice a day for a few minutes to release excess water vapor. It might help.