I recently received an email from David, a concerned pressure washer from Florida. He asks:
Our deck is in good shape except in a few spots, it has mildew. I was going to pressure wash it
then seal it. Is this a good idea?
Well David, if you have pressure treated pine as the surface of you deck , do not pressure wash it unless it is completely dirty and you have one of the smaller less powerful electric type washers. As they are less powerful, they generally wont have the power to do much harm.
If you have a larger (and higher powered) washer, you may easily damage the surface of your deck. It is possible to remove the soft wood in between the hard grain, and I have discovered that with pine you will coursen the surface, which leaves a very ragged appearance, and you’ll probably need to sand it afterwards. If you have soft spots or knots the high pressure might tear them up.
I use TSP (tri sodium phosphate – which you can get at any store with cleaning supply goods) and water with long handled scrub brushes. If your deck has heavy mildew, pretreat with straight commercial strenght bleach, wetting the wood prior to application.
You may also rinse with a high pressure nozzle attached to the end of a hose because you have hand scrubbed the surfaces.
Wait a few days, for the sun to dry out the wood a little then seal.