What is the best way to approach old deck boards with big cracks?

There is wood filler or use caulking

cool. Thank you.

DO NOT USE ANY DECK OVER PRODUCT!

There is a problem with the deckover products. They are known to crack and peel within two years. Hard to remove. Many Hometalkers have used them and regret it and have mention their misfortunes in discussions. Many people had o replace deck boards and even complete decks. There are class action lawsuits against the manufacturers. Also notice they are not advertised anymore. You can do some online research and find more cons than pros. I have been advising Hometalkers not to use the products and to spread the word. Some have tried scraping and sanding with fair results.

For temporary fix until you can afford to replace the boards or deck. Caulk may work because it is flexible. Wood putty may pop out with expansion and contraction. Bondo, auto body filler would be a better choice.

Don't use the deck paint that fills the cracks! It will bubble, peel and crack within 2 years and your deck will look terrible! You will then have to sand and scrape all the peeled and bubbled paint before you can paint again! I did this to my my old deck and really regretted that I had used that product. Doing that sanding and scraping is very labor intensive especially if you have a big deck! I will say that when you first use the paint type product on an old deck it looks good at first. Eventually we ended up tearing off all the old deck board floors and replacing with new. We were able to keep the supports so it wasn't the price of a whole new deck and we did it ourselves. You might be able to turn over the cracked boards that you have now or just replace those cracked boards with new. We will use a deck product that soaks into the wood rather than a product that sits on the top to keep the new deck boards looking good.

I did this to my parents’ old deck 3 years ago and you’ll need to power wash & dry those cracks, so that the caulk has something to bond to.

For the really deep ones, I would fill with a high quality wood glue, diluted a bit, so that it weeps down into the cracks.

If the boards are rotting inside those cracks I would first treat them with both a liquid pesticide and a fungicide. Wood rot, is a fungus and insects like to eat it.

I used Behr mold-killing deck wash and a scrub brush on the end of a long handle. I also used Behr’shighly acclaimed deck stain, in both translucent and opaque colors.

If you can under the deck, treat it as well.

Thank you all for your advice and sharing your experience with me.