My 1927 house has original solid wood doors with plywood panels. The old paint is "crazed" or cracked along the grain of the plywood on the panels, but not on the main parts of the doors. I have completely stripped the doors, sanded, primed and painted, but after about 6 months the cracking returns.
Until last summer the house did not have air conditioning, and I suspect the cracking may be due to expansion of the thinner plywood in the humidity. Could I be correct? Is there a way to treat the doors when I repaint them to prevent the cracking? Should I paint a door I haven't worked on before and see if our new a/c helps?
Re: Painting doors
You definitely have the right ideas about what's causing this cracking. Plywood products are kinda prone to this sort of affliction, but what determines how bad it might become depends upon the species of woods used to create the ply, how many laminations are involved, the thickness of the face veneer layer, exposure to varying humidity levels....etc. The "interior" layers/veneers are not exposed to climatic changes at the same rate that the outer layers are and so tensions develop within the laminated structure...often leading to surface checking.
Controlling the humidity level so as to keep it more uniform should be a benefit.....even though the older the wood becomes, the less elastic it is and the more prone to checking as a result......when/if the humidity levels vary.
I've refinished many vintage doors with ply panels that never had checked and didn't after the refinishing.....and some that were checked badly at the onset of the refinish. If you're painting these doors then you have a potential advantage in that you can use some common wood filler (Elmer's or similar) to fill those checks before repainting. That coupled with a "proper" prep & paint......and the new AC unit *may* do the trick. I would suggest filling the cracks, sanding that back flush, then *double-priming* with an oil-based primer or even shellac (Zinnser's SealCoat dewaxed product), then painting with either a quality latex or oil-based topcoat.
The OB primer or shellac coats will drastically slow the rate of moisture exchange between the environment and the surface veneer....and that should help minimize the stresses. (Latex primer is not nearly as good of a vapor barrier and so will not serve you nearly as well in this regard. Or at least creating this OB or shellac moisture exchange barrier has been my approach to the problem and it's served me well over the years.)
If it's any consolation, you're not alone. [url]http://www.sherwin-williams.com/pro/problem/problems/cracking_plywood/index.jsp[/url]
Re: Painting doors
You could well be correct in that swings in seasonal humidity might be cuasing your problem. However, rather than relying on yourt new A/C, the bigger culprit may be lack of sufficient moisture in winter. Do you have a humidifier? Without a humidifier, during very cold weather your inside moisture will get bone dry (in the teen's). It is the wild swing from super dry winter to humid summer that causes problems. You should try to keep the winter time humidity around 35%.
If you paint the panels again, I would try using a couple base coats of an 100% acrylic primer followed by a finish coat. I assume you have been using an oil paint as your finish coat. Acrylics are very flexible and have a better chance of bridging the grain of the plywood as it expands and contracts with the seasons. Whereas oil paint is not nearly as flexible as acrylic, I think it would adhere as an unbroken film to the acrylic coated substrate. I can recommend Behr's No.75 acrylic enamel undercoat as a primer.