We are about to stain a plywood beadboard ceiling on our new 14×20 screened porch. The ceiling is vaulted and has painted white beams/coffer so we are looking to get a dramatic look by staining the beadboard with a cedar-like stain with a polished finish. There will be minimal to no UV exposure or direct moisture exposure, but do we live in humid NC.
We have been testing different stains on the beadboard samples and Cabot’s Australian Timber Oil Mahogany Flame with a top coat of Spar Varnish looks awesome, but I saw several postings online where some people were saying not to put a poly or varnish on exterior stains (is this specific to Cabot?) because it won’t hold up due to the oils. What is the best way to achieve this look? Should I use an interior stain with a Spar Varnish top coat or will that cause mold/mildew issues? Cabot’s site does mention something about staining outdoor teak furniture with Australian Timber Oil followed by Varnish so maybe it is OK to combine the two? Thank you for any help!
Most "exterior" stains have a built in finish. If you are planning to use an "exterior" stain, DO NOT apply a top coat of varnish or any sort of polyurethane.
Exterior stains are stand alone finishes and will not allow good adhesion or proper drying of top coats.
When we have porch ceilings which require coloring, we use an interior stain for the color followed by a top coat of polyurethane. The label and the manufacturer will tell you to not use poly exterior. I have had good success with using poly exterior when I follow the practice of applying a fresh coat each year to ares in direct sunlight.
I have experienced spar varnish darkening considerably when applied over colors thereby changing the effect we were trying to create.
In your area, as mine, mildew is a fact of life. I have not had any negative effects of adding a mildewcide to a clear coat. The only difficulty is in mixing in the mildewcide as clear finishes do not particularly care for agitation. They will bubble. I get the paint store to use the "shaker" of the type which spins around on an axis while the can stands at about a 25 degree angle. This stirs the varnish as opposed to shaking.