You generally don't need sealant there. Just make sure the top is very clean and flat. I have a glass shelf from a refrigerator that I have stuck sandpaper to. I use this to flatten valve covers and anything else. You should only need to do it once if at all.

Old school sealant was Hi-Tack but that was mostly for cork gaskets. You don't want RTV floating around in your cooling system so don't go there. Visually figure out why it's leaking.

I bought the HF kit so I could pressurize the system without running to the parts store whenever I needed it but the kit can be borrowed from the store. I don't see the kit currently listed. There are several on Amazon. The SD uses the generic adapter so find an inexpensive kit.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.