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Try blending the scratches with medium then fine sandpaper first. Give the rubber seal a smooth surface even if not perfectly flat. Not sure what sealant would be gas proof though I'm sure you can find one at the parts store
The locking ring normally requires effort to compress a new oring.....I had to make a spacer ring out of 1/32" thick Teflon/nylon sheet material because of previous scratches/slight tank warpage.....get a lock ring tool to make the job much easier less chance of cocking or damaging lock ring.
Install......
O ring.....then tank sending assembly......then Teflon spacer ring......then Lock Ring. Then it took effort to rotate the lock ring until it reached it's turning limit......no more drips of gasoline after doing that.