Hi @KB3,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
In order for us to make an assessment on the best course of action, we really need to be able to see your deck. If you can grab some photos and upload them, that would be a great help. Let me know if you need a hand with this. I am happy to help if required.
If you don't know the exact product that was used previously, you will likely have some inconsistency in terms of how the new product soaks in, regardless of how well you clean it. So, it is best to strip the timber back to a uniform, uncoated surface before applying a new coating to ensure even coverage and colour.
Assuming the timber itself is in relatively good condition with minimal weathering, then you should be able to get away with using this Intergrain Liquid 8 Timber Stripper to remove the existing coating, then giving the timber a good clean with Intergrain UltraPrep Timber Cleaner and a Oates Long Handle Deck Scrub Brush before coating.
If there is any leftover coating or roughness on the deck, you can always use a random orbital sander or belt sander to smooth out the deck a bit before coating.
Check out How To Oil Your Deck and How To Sand A Deck for some further guidance.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
