Our home has wood framed windows on the inside and aluminum framing on the outside. The humidity build-up on the windows in the winter melts and drips onto the stained/varnished wood surface inside and bleaches or stains it. Yes, constant vigil with drying cloths does help but we would like to paint them white and, hopefully, lessen the maintenance on them. Can you suggest the best preparation and paint?
Moisture and condensation on older windows is a very common problem in our climate, during the heating season. I have addressed this issue several times, but your question is about a solution to the symptom, not the cause of the problem. I will answer this as well as offer one possible solution for improving the condensation problem.
Any older wood surface to be painted or stained and varnished must be properly prepared for the new finish to last. The first course of action is to remove any and all existing finish before proceeding. This can be done in one or more of several methods. The most common, but most labour intensive, is to scrape the existing finish and loose material off with a common paint scraper. These are inexpensive tools that can be purchased at any paint supply store or home centre. This is a long-handled tool with a curved, sharpened blade on the end. This blade is scraped along the painted wood, by pulling toward oneself, and removing layer after layer of finish. The blade can and should be occasionally sharpened with a file to maintain a sharp edge. The benefit of this tool is that it is inexpensive to use and the finish of the wood can be lightly scraped to improve the surface to be painted.
Another common method to prepare the surface is use of chemical wood strippers. These are a more costly and messy method of finish removal, but are often the easiest to use. They are normally painted on, allowed to soak into the finish until bubbling and then the old material is removed with a putty knife or paint scraper. Care must be taken to protect floors under the windows as well as surrounding painted surfaces. Gloves, long sleeved shirts and goggles should be worn to prevent contact of these caustic chemicals with skin or eyes, which can be quite painful.
A similar, but more environmentally friendly method for paint stripping is the use of a heat gun. These electric tools are specially designed for this application and will lift the old painted finish by heating it to a temperature that it can no longer adhere to the wood. The loosened and often bubbling finish can then be scraped off in a similar manner that that with chemical strippers. This method has its advantage over chemicals, because the cost is much less, in the long run. A one-time, moderate expenditure is required for purchase of the tool, as opposed to repeated payouts for cans of stripper. The heat gun should last for many years and may have other applications, such as replacing blades on synthetic hockey sticks or thawing frozen hardware. Care must be taken not to heat the old glass in the windows, as it could crack.
Whichever method of finish removal is employed, the wooden surface to be painted will require further preparation before going further. Once the majority of the old finish is gone, the wood should be sanded with progressively finer grit sandpaper until a smooth surface is attained. This can be done on large window sills with a combination power sander, if your own one, and hand sanding. This will help smooth out old nicks and dents in the wood, but further action may be needed if the wood is badly dented or chipped. Wood filler should be used to fill these defects after the initial sanding with rough paper. Once the filler is dry, a finer grade of sandpaper can be used to even out the surface of the wood sill and the filler. This will actually renew the condition of the sill and improve the look and feel of the final product.
Once finish sanding is completed and all the dust removed, the sill should be painted with an alkyd or oil-based primer. This is important because many woods, especially cedar and fir, which are commonly used for older windows, contain naturally occurring chemicals that can “bleed” through latex based paints. Sealing the wood with a proper primer will prevent damage to the new finish coat from bleeding. One or two coats of sealer/primer may be required depending on the condition of the existing wood. The primer may be lightly sanded with very fine sandpaper, after drying, to further smooth the surface. Once this is completed and the sill cleaned of all dust you are ready for the final step. The final paint finish may be either a latex or alkyd based paint, but should be high quality. Check with the paint supply store for the best paint available for this purpose and spending a few extra bucks for better paint will save recoating in the future.
To minimize the condensation in the future, if you are not considering upgrading the windows, air sealing around the windows may help prevent cold air leakage. One method that works very well with older windows, such as yours, is to install clear plastic over the inside of the window for the heating season. This material is inexpensive and can be heated with a hair dyer to remove wrinkles and prevents air leakage and condensation. The only drawback is that it is normally installed with 2-sided tape that may peel the new paint off the trim when removed.