Like most refinishing jobs, excellent PREPARATION of the wood surface is the KEY to success. Prep right, and the deck will turn out GREAT. Rush through prep, skip a step (or two) to save money and/or time, and the deck won’t look nearly as good.
Thorough cleaning of any deck or fence is essential to get good results. If the wood was previously sealed, the common practice is to strip off the old sealer. We do this to get back to the actual wood. All sealers are designed to bond best to the wood itself (rather than to bond to an aged layer of the old sealer). By stripping off at least the “coating” portion of the old sealer, we can eliminate most reasons for premature sealer failure.
If the “coating” portion of the old sealer remains on the surface, no part of the new sealer can penetrate down into the wood. This results in a couple of situations that could lead to premature failure of the new finish.
The first of these problems is a reduction of the permeability of the overall sealer layers. Permeability refers to the ability of the sealer to breathe. If we clog the surface with several layers of sealer, each layer comes a little closer to closing off the wood from breathing. In that case, the wood retains humidity. This could lead to rot, as discovered in a recent class-action lawsuit against a major manufacturer of sealers. This could also lead to peeling and flaking, as the trapped humidity tries to push its way up through the layers.
The second problem is bonding. As stated earlier, most sealers are designed to bond to wood fibers to some degree. Many sealers can also bond to themselves (such as a second coat). If we try to apply one sealer over top of another brand (or type) of sealer, there is no guarantee that the new sealer will bond to the old sealer. If this occurs, the new sealer could flake and peel off within months. Most sealer manufacturers are reluctant to stand behind the performance of their product unless it is applied to bare wood.
Further, sealers that are designed to penetrate, when forced to dry on the surface without penetrating, will fail by design. What I mean here is that a penetrant is not meant to stand up to wear like a coating does, so when you force a penetrant to act as a coating it fails. This happens when the penetrant can’t penetrate - and it dries on the surface.
There is at least one more reason to think about stripping every time you run across a previously- sealed deck. The reason you are being called in to do your magic is that the customer is unhappy with the performance and/or looks of the current sealer. Leaving that old sealer in place seriously limits how good you can make the deck look with a new finish. If you leave old stains, the new look won’t be much better than the old look.
For all of these reasons, seasoned professionals always strip decks that have old sealer on them. Exceptions are made if the Deck Tech was the person who applied the previous sealer and he or she knows exactly what the sealer is on the wood - AND the sealer manufacturer says it’s OK to recoat over an old layer.
How do you know if there is an existing sealer on the wood? Believe me, it is not always obvious. Cheaper products contain little or no UV protection, so a deck can look like a grayed-out neglected deck within a year of being sealed with an inexpensive product - particularly if it was a clear sealer. There are two things professional Deck Techs do to find out if there is any existing sealer that should be stripped off the wood. The first is to inspect areas that get no sun. A hint of color on the underside of a board is a sign of previous sealer. The second is really the acid test for existing sealer and is called the Splash Test. Sprinkle a little water from your fingertips on to the dry wood on a vertical surface. If the droplets soak into the wood right away, you don’t need to strip. If they form little droplets on the surface of the wood, then you have to strip the wood.