I have not personally used Blackjack but understand that it is good stuff.
You are considering painting it for your reason. There is an equivalent product available in white. You may consider it. Not sure if available locally for you at a home center. I have read posts where peeps used it with satisfaction. Here is a link to the product. Not sure why there is such a price gap from $80 to $240 for 5 gallons.
If you do a good job covering all cracks with the paint. A second coat where you suspect water can penetrate, and you should see good results. If water cant seep in , then you will not have separation. It is the water/moisture that causes the wood to swell and expand and cause us these headaches. Keeping water out any way possible is key.
PS. make sure you are painting wood that is thoroughly dry to start with.