Porch and deck floors are sometimes difficult surfaces to protect, as the softer a coating is, the easier it wears off, but the better it tolerates moisture transfer and wood expansion cycles. Conversely, the harder wearing a coating, the less able it is to handle expansion and contraction of the wood, so most coating choices represent tradeoffs to consider.
If wood is near the ground, as the sun beats down on the top surface, moisture is drawn from the cool ground underneath up through the bottom side of the boards. This causes expansion and contraction of the wood, and greatly reduces the long-term performance of the coating. Open ventilation on the sides will help, but if bottom sides of boards can be sealed, you will greatly improve the performance of the coating system used. If the wood is 2 feet or less off the ground, you should consider only a non-film forming wood sealer like Sikkens SRD. It is a premium stain sealer with trans oxide pigments that offer the best UV protection among stain sealers. This coating generally requires the maintenance coat every year or two, but because it is a non film forming product, there is never anything to peel, and it is slightly less slippery than some film forming products. I
f ventilation is good, at least two feet off the ground, and boards can be sealed on the underside, you could consider a thicker film-forming coating like Sikkens Cetol Wood Coatings. This is a three coat brush applied system. It is a little more difficult to apply, but on properly prepared wood surfaces, this coating gives practically a look of furniture, beautifully accentuating grain and color of the wood. This system requires the maintenance coat about every 3 years. This is a flexible breathable coating. You need to have the pores of the wood grain open for proper anchoring of the sealer. Also, any remaining previous coating will cause blotchy color and sheen variations in most finishes due to differing amounts of coating penetration on sealed versus bare sanded areas. The basecoat is the most important factor in longevity of the entire coating system.
Regardless of your coating choice, it will still be best to try to strip or sand old failed coating. We like Norton’s 60 g paper followed by 80G. Norton’s 3x series paper is also designed for coating removal. The paper really does make a difference. Most floor coatings will recommend 80 or 100 g finish sand, but it depends on the coating you choose. We use a Dust-free Palm Sander. If you have sharp corners on boards that could be slightly sanded round, that would be helpful. We always do a final brush vacuum prior to coating application, and actually wipe the surface to insure no dust can interfere with coating absorption and adhesion.