It will depend on what you want for a finished product. If you want to stain, you will need to use wood putty with the color you want to achieve. If paint then putty or filler will work. Durham's is probably your best best. Mix it up, smooth into cracks, then wipe with a wet rag or sponge.
How are you planning on re-doing them and the sizes of the gaps. You might need to pull the boards and reboard it. If its minor cracks, the word putty will work.
Thank you, I now have a plan!