Just got a new SUP. It's second hand and never had rail tape on it. It's in pretty good condition. Except one side has a small ding in it and it's been covered with a thick, hard gum. (Obviously to protect it). It's a pretty sloppy job but how do I go about putting rail tape over the top? I'm thinking it may not stick properly? Any ideas??

Thanks guys!

Take it to someone who will do a proper repair job, then rail tape it.

clean the wound: remove all the goop, do not hesitate to cut it free.
Then some solarez or similar and you are good to go. Rail or paddle guard, as you wish.

Be careful using turps or solvent to pull off old tape , I did that to an old PSH and ate the guts out of the board it seeped in through the dings. Heat it up with a heat gun and pick it of and keep rubbing with a rag , the glue balls up on the rag. At least a 6 pack job