Small checks and cracks: Ignore, fix, or replace
Over many years, if your redwood furniture develops a check or crack, it does not compromise the integrity or longevity of the furniture. If your set develops a crack or dent that has a rough edge, lightly sand the spot by hand for a few seconds to smooth out the rough edge. Use 100 grit sandpaper for a few strokes to remove any rough edges and then finish with a fine 220 grit sandpaper to match the smooth surface finish (redwood sands easily). After many years out in the weather, if your set develops a significant check, you can do one of 4 things:
In most cases, California Redwood furniture requires no attention at all for longevity's sake. For example, display items made of California Redwood remain out in the weather for many decades. One of them with a reputed California Redwood product manufacturing firm had developed a 4" long crack. According to that firm owner, it was a rare occurrence. It happened because every once in a long while a board is used to build an item that has built-in stress due to the way the tree grew (the tree could have had a bit of a lean for example). These relatively rare stresses are invisible when the set is built and occasionally appears many years later in the form of a crack. In such cases, their manufacturers recommend ignoring it and consider it a sign of "character".
But, if your crack has a rough edge, take a few minutes to sand it smooth. After sanding, if you want the sanded area to match the rest of the furniture, you can refinish the set (see "You can restore the original tones of your furniture in 15 minutes" above).
If the crack is significant or is just unattractive, you can send a photo to your manufacturer so they can make a replacement piece for you at minimal cost.
If you're a handy person and want to dabble in making the crack "disappear", you can either talk with your local hardware person for a product recommendation specific to your climate, or you can use a two-part epoxy (or five minute or high strength epoxy) available at most hardware stores. You'll need some masking tape and acetone (or nail polish remover that contains acetone). Clean around the crack and mask off the wood out two inches around the crack of the furniture no matter they are outdoor furniture such as picnic tables, arched pergolas, or indoor furniture like planter boxes, etc.
Make sure the masking tape is right up on the edges of the crack. Place masking tape underneath the crack to keep the epoxy from falling out the bottom or back side. Mix the epoxy and fill the crack, rubbing it in with your fingers diagonally. Before it has completely cured, remove the masking tape. Remove excess with acetone before it cures. Clean the epoxy off your fingers before it cures. Epoxy dries transparent and will last for many years.