The problem that you are having is probably due to the selection of the improper type of deck sealer last time. It sounds like you used a film-forming type of sealer that rests on the surface instead of one that penetrates the surface of the wood.
Film-forming sealers look nice, like a finish on furniture, but they rely on surface adhesion to keep them fixed to the decking. With very careful preparation of the deck before application, these film-forming sealers can be effective. In real life, though, there will be spots where it does not adhere adequately.
You will have to remove all of the film from the wood decking and start over again, this time with a penetrating type of deck sealer. Chemical-stripping chemicals made specifically for decks are your best choice. Choose a fairly cool, overcast day to strip the deck. It is not only more comfortable for you, but the stripper will work better, too.
There are several classes of strippers that will work well for your deck. They have varying degrees of strength and toxicity. The strongest chemical strippers typically contain a compound called methylene chloride. These strippers will work the fastest of all types, but care must be used.
Stripping chemicals that contain sodium hydroxide (you may also see alkaline on the label) have a lesser level of toxicity and work slightly more slowly. The mildest strippers use NMP, d-Limonene, or other milder chemicals. The labels of these chemicals may mention citric acid derivatives.
Buy the smallest package of one of the mildest strippers first and try it in a small, inconspicuous spot. Give it some time to work. If it is effective, buy a larger container of it and get to work. If not, move up to the next stronger stripper, and give it a try, too.
If you have trouble finding these strippers at your local home center store, here are some manufacturers to contact: Napiere for “Stripex” with a sodium hydroxide base; Rust-Oleum for “Wolman DeckStrip,” which contains mild chemicals; SaverSystems for “Stain Stripper,” containing sodium hydroxide and sodium metasilicate; Savogran Co.’s “Super Strip” with a methylene chloride base; and WM Barr, which has “Citri-Strip,” containing a mild citric acid base.
Once you have all of the old film-forming sealer stripped from the decking, you must clean the deck surface. Avoid all cleaners that have the word “bleach” in them, specifically chlorine bleach, which can damage the wood surface. A good pressure washing—not at the highest pressure setting—will clean it effectively and remove deteriorated wood fibers from the surface.
Now all you have to do is reseal the deck. A lightly tinted penetrating type of sealer will be most effective. It actually penetrates down into the wood fibers so that there is no chance that it can peel. Since you cleaned the deck, too, the natural graining will show through for an attractive appearance.
You will have two basic types of penetrating sealers to choose from: organic and synthetic. Organic sealers are less expensive and look good when first applied. The main problem is that the organic oils can support the growth of various molds and fungi that create dark spots. Synthetic sealers will not support this growth.
