C. R., Sharon, Mass.

A. Acid rain can certainly do a lot of weird things, including making stains. A chemical reaction, especially around the galvanized nail heads, is plausible. Try this: Apply a regular semitransparent stain (honey colored or as close to the color of the original finish as possible) over the yellow stains and see what happens. If that doesn't work, I can't think of anything else except to sand off the sealer and stain and apply a different stain.

Q. I am getting bids to build a deck in my place in Maine. One bidder proposes using cedar instead of pressure-treated wood for the floorboards. Is this OK? Will the cedar stand up in Maine or anywhere else?

L.L., Sharon

A. Yes, it is OK. The cedar will resist decay and certainly will stand up. But to do so, the cedar floorboards must be installed correctly: Before putting down the cedar floorboards, place a 6-inch-wide strip of roofing felt (tar paper) along the length of each joist, and fold it down on each side of the joists. No need to nail. Then install the spaced cedar boards in the normal manner. The strip of felt will act as a watershed, guiding water going through the gaps away from both the floorboards and the joists. With a little luck, the boards will last indefinitely, or at least much, much longer than without the felt.

This is an old-fashioned technique, which was used widely before pressure-treated boards were developed. But installers got out of the habit when using pressure-treated wood, because, they felt, it was not necessary. Now it is necessary when using nontreated wood, and is a good idea even when using treated wood. The technique may be old-fashioned, but it works.

Q. I would like to cover my deck boards so water will not leak down into the space below, which I plan to close in. How can I cover those boards and still walk on the deck? I can't build a roof under the deck because there are too many braces in the way.

CHARLIE MARIO, Lynn

A. A roof under the deck is best, but since you can't do that, here are some viable alternatives: Put down indoor-outdoor carpeting. This may not be watertight but it will hold water for a while, then allow it to evaporate when it stops raining. I have been told that the carpeting does not indent into the spaces between the deck boards.

Or put down fir porch flooring, which is narrow, tongued-and-grooved boards. Stain them with semitransparent stain. This flooring is called porch flooring because it has been used on porches for many, many years, and it stays watertight. Of course, the deck must slope a bit for this porch flooring to allow drainage.

The third alternative is to put down pressure-treated plywood and stain that with semitransparent stain.

Q. My son built a deck, then enclosed it as a room. How can he treat the ceiling of the crawl space under the floor? What kind of insulation would be good? One man said he would fill the space between the joists with R-21 high- density fiberglass, then staple Tyvek to the bottom of the joists to hold the insulation in place. Another said to foam in insulation.

HAL LEARNER, Newton

A. I think the high-density fiberglass is the best idea, and probably less expensive than the other. Stapling Tyvek (an airtight but vapor-permeable woven polymer that comes in rolls) to the bottom of the joists is also a good idea, although I suggest installing wire hangers to hold the insulation in place in addition to the Tyvek. And, be sure there is a vapor barrier touching the floorboards above before installing the insulation, if that insulation does not have a vapor barrier attached to one side.

Q. I plan to seal my 18-month-old, pressure-treated deck. A neighbor said he pressure-washed his deck before sealing it. We did the same to our deck, but another neighbor said we knocked off the preservative in the wood by power washing. Is that really so? If so, now what? Should we seal it as we planned?

M.A., Milford

A. Relax. Pressure washing did not wash off any preservative in the wood; the preservative is there to stay. The most the washing did was to take off any preservative left on the surface, and that is unlikely because weather did that over the past 18 months.

So, what now? You can put a water seal on it, being careful that it is warm enough (55 degrees or higher) when you apply, and for two days afterward. Or leave it alone, till spring. Then put on the sealer or a semitransparent stain. I personally prefer the stain because it will give the deck a color, only one coat is needed, it will last up to five years, and is easily reapplied. The sealer does make the deck water-resistant, but usually requires recoating every one or two years.

Q. My new deck is floored with 1-by-4-inch fir boards. Is there a need to protect them? If so, with what? Can I let them weather?

A.A., Chilmark

A. You should protect the fir; it will not weather well and is prone to decay. There are two ways to do this: 1. Use a clear sealer, which has to be repeated every year or two. Such clear sealers will allow the boards to weather, which is OK in that case. 2. Apply one coat of a semitransparent stain. No second coat is needed, and the stain must be reapplied every five years. When recoating, only one coat is needed.

Q. I am replacing deck boards that are not pressure-treated; the joist and other parts of the framework are. How far apart should the boards be? How far apart should the boards be end to end?

GERALD MARTIN, Dorchester

A. Space the boards 1/4 to 3/8 inch. I think 3/8 is better because when they swell with moisture, the boards will not close the gap. With 1/4-inch gaps, this is possible, thus preventing proper drainage. For boards meeting end to end, spacing of 1/8 inch is enough.

Incidentally, when putting down the new boards, first lay a 6-inch-wide strip of tar paper along the full length of each joist, and bend the strip down on each side of the joist. There is no need to staple or nail this strip, but it should go on all joists. The strip acts as a watershed, and will help keep the floorboards (and the joists) dry. With this funny little technique, the boards can last indefinitely, because when they get wet, they will dry out. It works.

Contractors got out of this habit when pressure-treated wood was developed, but they shouldn't have. It is a good idea whether pressure-treated wood is used or not. One good thing about the strips is that they are not expensive and it takes very little more work to install.

Q. My deck is painted with a deck enamel and it's a mess, with peeling paint and bare spots. I got the idea of turning over the square-edged, 1 x 4 fir boards and re-laying them with slightly wider gaps between boards. They seem to be too close together now. Would this work?

DAVID ROSTOV, Acton

A. It sure would, and congratulations on coming up with a practical solution to a miserable situation. Pry up the boards carefully, so you don't split them. If the boards were installed with galvanized screws, the screws should come up easily. If the fasteners are galvanized nails, the job is tough, because galvanized nails hold better than any nail.

Once you have the boards off, scrape or plane off any paint that got on their edges. Reapply the boards, and space them 3/8 inch. Stain with a semitransparent stain; only one coat is needed, and you will have to repeat (oh, my!) every four or five years. But no peeling, and each application of such a stain needs only one coat.

Q. I'm fretting over whether to use mahogany or cedar for the floor boards of my pressure-treated deck floor that has rotted out. What is best?

M. P., North Andover

A. Mahogany vs. cedar: It's a toss-up; both are resistant to decay. Both will weather gracefully without treatment; cedar will darken more than mahogany and is softer than mahogany, so will show more wear and tear marks. Let's put it this way: If I had the choice, I would choose mahogany, although I just might say to hell with it and choose pressure-treated wood; your pressure-treated boards that rotted out I don't think were really pressure-treated. Some years ago, a lot of wood was stained green to make it look like pressure-treated wood. Many people were taken by this scam.

At any rate, whether you choose mahogany or cedar, the way you install those floor boards is most important. Done right, the boards will last many years, if not indefinitely. And the right way is to lay a 6-inch-wide strip of tarpaper on each joist, lengthwise. Fold it over each joist so it is slanting down on each side of the joist. Then install the boards with a three-eighths-inch gap. The tarpaper acts as a water shed, with water running between the boards and down the sloping tarpaper, out of harm's way, giving the board a chance to dry out after it stops raining.

This was done routinely before the invention of pressure-treated wood; in fact, lumber stores carried 6-inch-wide rolls of tarpaper just for this purpose.

When pressure-treated wood came along, builders got out of this habit because it was not needed any more. It is still a good idea, whether you put down mahogany, fir, cedar, pine, spruce or even pressure-treated wood.

Q. I'd like to paint the railing and spindles on my deck, and I know that paint could peel. Could I thin down an oil paint, which would make it sort of a stain?

L. C., Framingham

A. Yes, because all stain is thinned-down paint, and the thinner you make it, the less it is likely to peel. You will have to experiment with mixing the paint with solvent (paint thinner or turpentine) to get the right proportion of paint to solvent. Make a small batch, keeping track of the proportions, and apply it to the railing, let dry and see if you like the color. Once you have the right color and consistency, do the whole railing. You can do the same with latex house paint, using water.

You could save time and labor by buying an oil-based semitransparent stain. In either case (your homemade stain or a store-bought stain), only one coat is needed.

Q. I plan to stain my deck that already has a deck stain on it. The present stain has a matte finish. Does that protect the wood? What can I use to make the deck shiny?

D.D., Brighton

A. If you want to restain, continue with the deck stain, but you may not have to do it for a while, depending on what shape it is in. If it is in good shape, forget about it for a while; it does protect the wood, just as any semitransparent or deck stain will. If it is beginning to look a little worn, wan, or just blah, you can probably restain now with the deck stain.

Do not try to make the deck shiny; anything that makes it shiny is either paint or varnish. The paint will peel, increasing your maintenance chores. And the varnish will peel or fail even faster than the paint.

Q. The cedar floorboards on my deck are very nice, except for one thing: a whole bunch of little resin drops oozing out of the cedar. How can I get rid of them and keep them away?

B. M., Holliston

A. The drops are pretty obviously cedar resin, but any softwood, including pine, fir, cedar, juniper, spruce, and others, will exude those resin spots, which is essentially congealed sap. If they are hard, scrape them off with a chisel. If they are soft and sticky, remove then with rubbing alcohol or denatured alcohol. You cannot keep them away; neither can God. No sealer will prevent their coming through. Live with them, and deal with them as they appear. They might just go away some day, but don't hold your breath.

When C. W. of Framingham discovered a small hole in the smokepipe of his newly installed heating system, he asked what it was and why it was there. He was told that it is a hole for inspection purposes and is safe. The handyman confirmed this, wondering how a visual inspection can be made through a 1/2-inch hole.

M. D. of Somerville, who is a licensed oil-burner technician, wrote to confirm this as well, adding that the funny little extra hole is probably safe. ``Its purpose is for the insertion of a sensing probe, . . . not for visual inspection. The proper size is about 1/4 inch.

``Through exhaust readings taken at this point, the service person determines the most efficient burner adjustments and the proper draft for your heating system. These tests also assist in finding problems which need to be remedied but are difficult to `see.' These include but are not limited to chimney or boiler blockages, excess draft, and combustion chamber deterioration.''

Q. I am building a deck that gets a lot of intense sun. I think I will use pressure-treated lumber for the frame and mahogany for the floor. For a white color, what should I use for the railings? Should a tarpaper strip be used under each joist? How far apart should the boards be -- an eighth of an inch? How should they be applied -- nail, screw or clip?

R.H., North Andover

A. Your choice of wood is good, in the Handyman's opinion. For white, paint the railings with a solid-color latex stain; one rule says you are not supposed to use a solid-color stain on a horizontal railing, but you can make yourself a guinea pig and see how long it lasts. This rule applied to floors originally, where there is a lot of foot traffic. For a rail, it is worth trying; it could last up to five years.

To apply, use clips if possible. There will be no screw or nail heads showing. The tarpaper, a 6-inch-wide strip laid lengthwise on top of each joist under the boards, is a good idea. It is old-fashioned but is effective in preventing decay because the tarpaper strip, folded down on each side of the joist, acts as a water shed.

Spacing of an eighth of an inch is not enough. Better is a quarter of an inch, and three-eighths of an inch is good, too. The eighth-inch spacing is not enough because the boards can expand as they gain moisture, closing the spaces and preventing drain-through.

Q. I do a lot of decks, but one I did recently is squeaking like crazy. The deck boards are cedar, applied to pressure-treated joists with stainless steel screws. The pressure-treated wood was green (full of water) when I put on the floor boards, and the joists must have dried out and contracted. It's funny; when it rains there are no squeaks.

JEFFREY JOBST, Athol

A. It's an old story: The joists dried out and shrank, as you said, and the screws lost their grip. When the joists get wet (when it rains) the wood swells and the screws grab again. Solution: Wait for dry weather (so the joists are driest and smallest), and move the screws 3/8 of an inch to one side. Don't move them too far because they won't go into the joists.

A more permanent solution is to take up the boards (horrors!) and relay them, using construction adhesive to hold them down. Then drive the screws in their new locations. Or, if you can find them, use deck clips instead of screws. These clips secure the boards nicely, but with no nails or screws showing.

Q. I had a new deck of fir floorboards installed, and the contractor said not to seal them. Is this correct?

MARIAN IRVING, West Tisbury

A. I think it is not correct. You can seal any wood outdoors, but like most sealing projects (on cedar, pressure-treated or mahogany) such boards should be sealed every one or two years, which proves to me the sealers aren't very good. It would be better to stain the boards with a semitransparent stain; at least such a stain does not have to be reapplied for three to five years.

Q. I got three bids to build a 16- by 18-foot deck. One builder suggested installing a concrete pad four feet below the ground and mounting wood posts on them to hold the deck. The others suggested Sonatubes, concrete cylinders set 4 feet into the ground with their tops 6 inches or so above ground. Then posts are set on those cylinders. What is best?

R.P., Milton, Mass.

A. I think the concrete cylinders or piers are best. Setting the pads 4 feet down, then putting the posts on them, doesn't make sense to me, because 4 feet of the posts will be in the ground anyway; you might as well put pressure-treated posts 4 feet into the ground without the pads, which would be OK anyway. The reason everyone is talking about putting things 4 feet into the ground is that it brings the base of the structure below the frost line, thus preventing heaving and other movement from freezing and thawing of the ground.

Q. I bleached my pressure-treated deck last summer with a Thompson's bleaching material. Does bleach tend to reduce the water resistance of the wood? If so, what can I put on the deck?

JOHN HAMBLIN, Revere

A. Bleach is unlikely to reduce the water resistance of wood, particularly pressure-treated. If you want to increase the water resistance, apply a clear sealer or semitransparent stain. The sealer should be reapplied every one or two years, the stain every five years.

Q. My deck was stained black -- on purpose -- but in the summer it is just too hot on my feet, and everyone else's. Can I stain it a lighter color? The black stain was applied two years ago.

P.S., Hudson

A. Ah, yes. Remember when we'd go to the beach and be very happy to walk on the sand on a hot, sunny day, and would be very happy to stay off the asphalt, or try for a concrete rather than an asphalt road or walkway? That experience, and yours, simply proves that dark colors do, indeed, absorb heat.

OK, for the nitty gritty: Normally you cannot stain over an existing stain because the existing stain is like a sealer, making the wood water-resistant, and the wood will be resistant to the penetration of a new stain as well. But since the stain was applied two years ago, enough might have worn and weathered off that you can apply the lighter stain. If the new stain beads up on the old, then you will have to wait a year or more to try again. And, since the old stain is an actual stain, it is likely to last longer than two years.

The only alternative to living with the black stain is to sand it to the bare wood, then apply the lighter stain. If your deck boards are pressure-treated, you are not supposed to sand it, but if you take up the sawdust as you sand, and wear protective clothing and a respirator, it should be OK.

Q. What's the recipe for cleaning a dirty deck? I have seen so many on the market that I am confused.

B.A., Lexington

A. Sometimes a simple mix of one part bleach and three parts water will clean up a deck nicely. If not, make a strong solution of TSP cleaner and water, and add a cup of bleach to the bucket. Or, substitute Spic and Span for the TSP. Some people warn against mixing bleach and TSP, but it should be OK outdoors. Always wear skin and eye protection.

Makers of decks, pressure-treated wood and sealers for such wood have come up with a wide variety of cleaners, but the Handyman has found them too mild to be effective.

Q. I want to put a preservative on my deck that will dry properly and not be tacky. I get a lot of fog and dew in the mornings, and because of this moisture, past coats never dried. What can I use?

GLORIA DEEG, Sharon

A. Sometimes a sealer or preservative doesn't dry because it is applied too thickly. The cure is to sand it off or remove it any other way, and apply a single thin coat. Another reason for tackiness is a sealer applied too soon after an original coat. Generally, only one coat of sealer is needed, but has to be redone every year or two, which doesn't make sense when you're trying to maintain a deck.

A better finish for a deck is to remove the old finish and apply a semitransparent preservative stain of the color of your choice; if you can't find a preservative stain, get an ordinary semitransparent stain. One thin coat will last up to five years, depending on how particular you are.

Q. The pressure-treated floor boards of my deck are 11 years old and are rotten. Can I turn them over to expose undecayed wood? How can I pull them off? The nails seem very tight, and I might pull their heads off.

JOHN ELWOOD, Bourne

A. Pressure-treated boards are not supposed to rot, and, in fact, do not. I suggest the boards are not pressure-treated. Besides, if the boards are rotting at the top and/or sides, turning them over will do no good.

So, tear them off and install properly pressure-treated 5/4 boards (a full inch thick). Pressure-treated boards are identified by a blue, yellow, or red plastic label stapled to one end.

Tearing them off might still be difficult. First, check to see if the fasteners are screws. If so, back them out with a power driver. If they are nails, use an 30-inch-long pinch bar rather than the standard 24 inches. Chip out wood around the nail head so you can get the hook of the bar under a nail head. Pry steadily. The extra length will give more prying power. And, put a block of wood under the fulcrum (where the curved part of the hook rests on the floor) to give even more pulling power.

If that doesn't work, tear off the floor boards, leaving the nails exposed; if you can't pull them, cut them off with a hacksaw.

Q. We were told by the builder to put a sealer on our beautiful new deck. We did, and a painter suggested putting on a second coat, which we did. It was a mistake. The second sealer did not penetrate, and we had a sticky mess. We were told to use a sealer remover to take everything off, but now it is a patchy, sticky mess. What can we do?

ROSEMARY RAGSDALE, Charlotte, N.C.

A. Yep, it'll happen every time: Two coats of a sealer will result in a sticky mess, just as a coat of linseed oil left on without wiping off will stay sticky forever. Obviously, the painter who suggested a second coat was wrong. The sealer remover (whatever that is!) also didn't work.

What to do, what to do? Remove everything with chemical paint remover, either the methylene chloride remover such as Stripeeze, or 5F5, or a citrus-based remover such as Citristrip. The Stripeeze or 5F5 may do a better job, but I think, in case of the sealer, the Citristrip will be effective and safer to use.

The alternative is sanding, which is OK if the deck floor is not pressure-treated boards.

With everything off, you could leave the deck alone, if it is of pressure-treated wood. Or, pressure-treated or not, apply one coat of sealer; it will have to be renewed every year or two. Sealers are way over-touted; sellers of such sealers never say you have to renew every year or two; another maintenance job.

Better yet, in my opinion, apply one coat of semitransparent stain. This will give good color (of your choice) for two to five years. When it begins to wear off, apply another coat, but if the second coat tends to bead up or not penetrate, stop, and wait for the first coat to wear off more.

Do not use paint or a solid-color stain, or a so-called deck stain. The latter is new on the market, with great claims and promises, but it, too, does not work because it will peel. Semitransparent will not peel.

Q. I am having a new deck built and like the idea of Trex, a board made of ground-up wood and plastic grocery bags. The contractor is not thrilled with the high cost of Trex, but I still like it. The deck frame is wobbly and quite high off the ground. Would the Trex floor boards help make the frame more rigid?

D.P., Northborough

A. Yes, they would, but they should not be used for this purpose nor for framework because they are not structural, not strong enough, so make sure the framework is made of pressure-treated wood or standard lumber. You should stiffen the framework with cross-bracing or other method before putting on any floor boards. Then you can use the Trex for the floor.

Q. I am replacing the spruce floorboards (they rotted out) with pressure-treated boards in a 10- by 40-foot deck. I think the joists and other parts of the frame are OK. Should I put the new boards lengthwise and parallel to the house, the way the original boards were, or should I put them at right angles to the house, so I can use 10-foot boards and don't have to butt any boards together, end to end.

THEO MADDEN, Duxbury

A. Put them the same way as the original boards were; keep a 3/8-inch gap where they butt end to end, over a joist. If you tried putting the boards at right angles to the house, you would have to rebuild the frame.

The only problem you might have is water staying in that joint where the boards butt end to end over a joist. In fact, that is where decay might have occurred in the frame. To get around that, I suggest you lay an extra joist an inch or so away from the original joist, wherever boards butt end to end. That way you can have the gap between board ends, but the water will drip into the space under the deck, preventing further decay.

Q. The back deck is pressure-treated wood, but never was sealed. Should it be sealed, or should I treat it with semitransparent stain that you seemed to prefer.

BEN SIMON, Somerville

A. A pressure-treated deck is sealed with a clear sealer such as Thompson's to keep the wood from splitting and spalling (bits of wood and slivers coming out of the wood). A sealer generally has to be renewed every year or two, adding to the maintenance chores of a deck. If your deck boards show no sign of splitting or spalling or misbehaving, you don't need a sealer. But you're right that the handyman prefers semitransparent stain; this will do the same as a sealer, provides color, needs only one coat, and will last a lot longer, three to five years.