I'd suggest cleaning the threads with something other than gas, maybe carb cleaner or something similar. Should clean up easily. A tooth brush works. Sealant can be applied with your finger or maybe a Q-tip I guess. I would not be too worried about the old sealant getting anywhere, there is not much of it and it cleans off easily. Once you apply the sealant to the male and female threads (or just male threads really) let it set up for maybe an hour. It should get a little tacky. It won't "dry" or harden really, just congeals a little. Then hand tighten the fitting and follow that with maybe 1/4-1/2 a turn with a wrench. DON'T OVERTIGHTEN PLEASE!
Hope this helps. We have had very few issues with guys installing these but sometimes stuff happens and the sealant does not work as it should. The Permatex sealant I suggested as an alternative is not permanent and seems to be a thicker consistency for those that havetouble with the Gasolia we supply.