Although many products for filling exterior wood are available, not all are suitable for every purpose. Exterior spackling compound may be suitable for filling nail or screw holes, but it won't last if you use it to fill unstable cracks or rot and fungus pockets. Some products will bind to wood and keep cracks from getting worse, and others will penetrate and make the wood stronger.
Reasons to Fill Deck Wood
If you've just installed your deck, you may only want to fill nail or screw holes, but if the deck has been standing for a while, you may have more extensive defects to repair. Common problems caused by weathering and wear include cracks, gouges and pock marks. Some of these problems need to be corrected before you apply filler. A crack can continue to spread if measures aren't taken to abate the spreading, and pock marks can signify a fungus infestation that needs to be removed.
Conventional wood fillers for interior woodwork won't work on your deck. They are usually formulated with latex or wood fibres bound with an adhesive and are not designed for exterior conditions. They shrink in direct sunlight, do not respond well to wood movements and do not resist moisture. A repair made to your decking boards with interior woodwork filler may look good after you've finished the repair, but after a month or so, you will have to do the repair again. Products not suitable for filling decking boards include hardwood floor filler, interior spackling compounds and coloured wood fillers for cabinets and woodwork.
Types of Fillers for Deck Wood
An exterior wood filler has to either remain flexible so it can adjust to movements in the wood, or it must be able to bond to the wood and prevent the movement. Exterior spackling compounds retain a putty-like consistency and don't harden the way interior ones do. Epoxy-based wood fillers have excellent adhesion and become harder than the wood itself when they set. Some fill gaps like putty, while others penetrate the wood like a wood sealer. These penetrants strengthen the wood by binding the fibres.
You can spread exterior spackling compound into minor defects with a putty knife, but do it carefully. Because it doesn't harden, you can't smooth it or remove the excess by sanding. Epoxy filler comes in two parts that have to be mixed together before you can spread it with a putty knife. You can sand it, but it is very hard, and if you patch a large area, the process can be tedious. Depending on the consistency, you can paint epoxy penetrant with a paintbrush or spread it with a putty knife.