Our outdoor porches have regular drywall ceilings mildewed and losing paint from damp: how can I clean and coat/paint it to protect it properly? I’m trying to avoid replacement.
Prime it with an oil based primer sealer, let dry thoroughly and finish with an exterior latex paint.
You can cover the drywall in the ceiling with wood siding prim and paint.
It’s will protect and it will look good.
Consider using a block and wall filler that contains elastomeric liquid rubber. It should protect the drywall better than paint and similar materials.
If drywall desired a quality outdoor paint a primer will do, otherwise consider replacement with an outdoor product application
To be honest with you…Tear down the drywall and install or have installed an exterior panel or boards that are designed for cold, wet or damp weather. Otherwise you will be facing the same problems in the future with moisture getting behind the product installed and creating mold that you do not want. As well as weakening the ceiling structure.