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First, don't use latex paint. Latex paints are a marvelous invention for many reasons but, in some applications and under some conditions, gloss and semi-gloss latex paints can develop a "tackiness" that seems to last forever. I painted a garage door with an exterior latex paint over 5 years ago and, in the warm weather, I still hear the sound of the tacky paint breaking contact as the door goes up!
The problem you're having is known as "blocking", and can be caused by many factors such as applying an overly thick layer of paint, not allowing adequate drying between coats, reactions between the primer and the finish coat or the temperature at which the paint was applied... too hot or cold.
The best way to avoid blocking is to use an oil-based exterior alkyd paint instead. Alkyd paints, which are the very top quality oil paints, dry hard and smooth and do not produce the tackiness you have been experiencing.
To eliminate blocking, the only solution is to either (1) remove the paint completely and start over or (2) prime with a quality latex primer sealer, allow to dry thoroughly and then reapply the finish paint in thin, even coats with proper drying time between coats.
Concerning your painting intentions, there are mixed views among painters on preparation for a latex-to-oil changeover. In my experience, oil paint can be applied over latex without priming as long as the environmental conditions are not too demanding. Of course, a light sanding OR the use of a "deglosser" such as Wilbond is essential for a firm paint bond.
In the case of an exterior door, though, I vote for (at the least) sanding followed by priming the doors prior to applying the oil. The primer must be an exterior grade and can be either oil or water-based. Again, there are some strong opinions on using either type of primer... just be sure it is an exterior grade. Under NO circumstances use Kilz, BIN, or any of the other interior primers.
Doors that are protected by an exterior storm door can have unique problems. Manufacturers of metal exterior doors generally frown on the use of storm doors, since the air space between the metal door and storm door can become hot enough under exposure to the sun to blister paint and distort the plastic moldings that some companies use to mount the window glass! This can also occur with wood doors, causing the paint to prematurely fade or appear weathered. In fact, this overheated situation can even cause wood doors to expand and contract severely enough to cause cracks in the door panels! And guess what makes the situation even worse? You guessed it… dark pain colors!
A final comment concerning blocking… if you are not ready to repaint and would like to eliminate the tackiness, one method that I have used successfully is to apply auto or paste wax to the sticky area. The wax coating removed the tackiness, though it may have to be repeated occasionally if the tackiness reoccurs.
This is one of the risks I take every day when I put my paint-stained fingers on my clean, off-white keyboard and try to answer questions from readers like "J" in the April newsletter. Because of the nature of the medium, I rarely have enough information to make a complete appraisal of the reader's problem. Based on the information I am given, I make assumptions and from those assumptions I try to formulate an answer that seems "at the time" to be reasonable and complete.
Unfortunately, as you point out, I may have "missed da boat" on this one. I read "stick" as "sticky"... you read "stick" as meaning "stuck"... or jammed, rubbing, etc. In what is admittedly 20/20 hindsight, I realize that J probably read my answer, shook his head and said to himself, "What medication is this guy on?"
Sealing all edges of the door as you recommend is the correct way to limit or eliminate swelling in wood doors. Indeed, ALL manufacturers of wood doors require sealing on all edges or they will not honor their warranties. Some exterior doors come pretreated with waterproofing to give you a fighting chance against swelling. However, if any trimming or cutting of an exterior door is ever done, paint or a clear waterproofing should be applied to the cut area.
Sealing should be done when the doors have the least amount of moisture in them... the dead of winter is the ideal time provided that you can keep the door open and warm enough for the paint or sealer to dry... if you have a good storm door, for example, or if you are willing to seal the entryway with plastic sheets. Otherwise the next best time would be early spring while the temperature/humidity index is still low. Paints and waterproofers usually have a low-temperature recommendation for their use that should be followed.
If the door were to rub ALL year, it is necessary to either cut or sand the rubbing edges as necessary to allow the door to close easily and to make room for the thickness of the new paint… with a little extra for luck! It is a fact that a bit of seasonal movement will occur in the door frame regardless of the amount of door-sealing you do and that, many times, sticking doors are caused by frame expansion not door expansion.
I never heard of corn starch to eliminate "blocking" (surface stickiness in dried latex paint) but what the heck... if it works do it! A few other folks contacted me and recommended using talcum powder instead, which might be superior if only because it is chemically inert (talcum is a mineral, not organic) and won't attract any vermin.
Regarding paint primers, I have been in the business long enough to remember when both BIN and KILZ recommended against using their "flagship" products on exterior surfaces except for very small applications such as knot sealing. Kilz has developed both water-based and oil-based clones that are specifically designed for interior or exterior use, and I tend to go with the manufacturer's recommendations unless I know for sure that they are "blowing steam".
I have talked about this problem before and I am sure that, until the latex paint manufacturer's figure out a way to solve it, I will be doing so again!
Latex paint, due to its chemical nature, can retain a certain amount of "stickiness" for a long period of time after it dries. This residual tack is known in the latex biz as blocking. Though all latex paints, interior and exterior, have anti-blocking agents added to them, the final results are mixed. How much blocking you will experience with a given paint job is related to various factors… the number of coats you apply, the thickness of the paint coats, the temperature and humidity in the room, how well you mixed the paint, and the manufacturer.
Blocking is not a consideration with walls and, in most circumstances, doors and trim. One problem is that exterior doors painted with latex paints are notorious for sticking to their weatherstripping. I always advise against using latex paints for surfaces that will be in regular contact with anything. In sympathy with your plight, I too have seen books lightly sticking to latex-coated shelves after years of drying!
Though there are "prophylactic" solutions, such as waxing the shelves or dusting with talcum powder, the best and most permanent solution is to repaint the shelves. The paint of choice for shelves and cabinets is a high quality alkyd paint. Alkyd is the best of the oil-based paints, drying to a hard, non-tacky surface every time!
My advice is to lightly sand the shelves with a fine (220 grit) sandpaper, then give them a complete coat of a fast drying primer… you can use a latex PRIMER such as Zinnser 123 if you want. The primer is important… alkyd oil directly over latex FINISH PAINT is not advisable. Then apply one or two coats of a matching alkyd paint in either gloss or semi-gloss. If you are a careful painter, you can probably just paint the top surfaces of the shelves… it will save you some work! Once dry, your sticking problems will be gone forever.
Your problem is common but nevertheless frustrating. Because door panels expand and contract at a different rate than the rails and stiles... the horizontal and vertical components of the door... annoying paint "gaps" appear around the perimeter of the panels. This expansion and contraction is not really due to temperature changes but instead due to seasonal increases in the amount of moisture in the air… humidity… which causes unprotected or inadequately protected wood to swell.
There is no surefire way to solve this problem but it can be minimized in a NEW door by completely sealing the door prior to painting with a "paintable" wood sealer. A paintable sealer will give painting recommendations on the label. Be careful to read the label as sealers have special painting requirements, such as the use of an oil-based primer. Others have minimum waiting times between sealing and painting. Don't use a deck sealer because many are so rugged that they require actual weathering before painting should be attempted! Most wood door manufacturers seal their exterior doors before shipment to your local lumberyard, but this is rarely done for interior doors.
Since your doors are already painted, the most complete but also most laborious solution would be to totally strip them, seal them and then repaint. As you know from experience, repainting alone is not a solution since paint cannot creep into every crevasse that a liquid sealer can.
As a less expensive alternative, you could seal just the tops and bottoms of the door with a clear wood seal. These areas are often overlooked in door protection, especially with interior doors, but are important in stabilizing the amount of moisture in the door. In fact, most manufacturers void their warranties if you don't seal ALL edges of the door against moisture... interior and exterior! If you visualize how moisture moves through wood, you can see why the top and bottom edges of the stiles would absorb large amounts of moisture!
MDF is an acronym for "medium density fiberboard". It is a wood product made from wood fibers and bonding agents and can be pressed into boards, machined into decorative moldings or pressed into shapes such door "skins" (faces) that resemble real panel doors. MDF is currently used for moldings and cabinet doors, though it is being used more for interior doors. Its most endearing qualities are that it resists cracking, warping, shrinking and swelling.
However, MDF is not the only man-made material that is used for doors. HDF, or "high density fiberboard", is a sister product that is more commonly used for the "skins" or faces of a special type of interior door called a "moulded" door. Moulded doors are available with either flat or panel-like appearance in many styles. They are also available with hollow-cores or solid-cores, though you may have to special order the solid core ones. Solid doors are much heavier and thus more sound deadening and... well... solid! Hollow doors are much less expensive and lighter in weight, though this makes them more susceptible to damage by angry (pick one... men, women, teenagers, pit bulls).
Because the moulded skins are not a wood veneer, they are unsuitable for staining and are sold pre-primed in what we in the business call "paint quality". Expansion and contraction does not affect the paint film in moulded doors since the faces expand uniformly and minimally. And I think they look rather nice considering the low cost and low maintenance as compared to wood doors. I have seen them in both thrifty and palatial homes. In truth, once one gets past the snob appeal of a real wood door, the benefits of these manufactured doors can't be denied. Because they are made from wood and wood byproducts and not valuable and scarce hardwoods, they are also an environmentally wise choice.