There are discussions pro and con for both surface preparation methods. Our experience suggests that both methods are best when used together. We prefer to ask, “Which cleaning method should I use first?”
Assess the Condition of the Wood
We assess the condition of the wood. This will determine whether sanding should preceded washing or whether it should follow it.
- Splintered, cracking, or just plain rough wood should be sanded first, then pressure washed to open the wood’s pores and allow penetration of the stain.
- Deeply grooved wood that has mold, moss and other growing things filling the cracks will need treatment with a deck and siding cleaner. Then use a stiff plastic bristle brush to loosen the debris. (Always work in the direction of the grooves/grain.)
- Smooth wood that is in fairly good shape except for fading and wear in the finish can be pressure washed first. After the deck has dried follow with a light sanding to knock down any fuzz or raised grain caused by the washer.
Identify the Type of Wood
We also consider the type of wood. Some popular deck woods, such as cedar are very soft. You can’t be zealous with either sanding or pressure washing. Pressure washing tends to fuzz up the surface on cedar, so you’ll need to be sure you use a lower pressure setting. Then follow up with light sanding to smooth the surface.
If your deck is made of pressure treated wood, you might not want to pressure wash at all. It releases arsenic into the water and contaminates anything the spray lands on.
The Environmental Working Group recommends cleaning your check with soap and water. Commercial deck cleaning solutions or TSP can convert the chemicals in the wood into their more toxic forms.
Sanding is also dangerous as the dust contains arsenic. So you only want to sand dangerous splinters. Use a vacuum capturing sander to reduce the risk that you or your pets and children will be exposed to toxic chemicals while you refinish your deck.
Author:
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5 Responses
I have a wooden ceiling over my patio. If I power wash it, do I still need to sand the wood before applying poly on it? Thank you.
Depends what is on the wood now and in what shape. Raw wood doesn’t need sanding in this situation. If the ceiling has a past coat of clear it will need a light sanding to help with adhesion.
They took arsenic out of pressure treated wood preservatives in 2003. So unless your deck is 20 years old you can sand safely
In the case of rough older wood on a deck that qualifies for your above recommendation of sanding then powerwashing, will the surface stay smoothly sanded after being powerwashed? Can the stain/sealer go on after the powerwashing?
Yes, as long as you don’t try to “strip” the wood. Example; Use the wand with tip close to the surface. Use the washer to just rinse off the dust and open the pores. Allow the deck wood to dry before applying the new stain/sealer, typically just wait overnight.