Jim,
Just like you did, I ordered the stopper that was recommended for my 6 gallon vadai on MoreWines web site. It was too large and had a breathable air lock in it for barrel fermentation. I had to order the silicone bung for small barrels. It also set me back a week.
I used a spray bottle with kmeta on the spots and blotted the excess with a clean rag. Works great.
The reason I had so many stains on my barrel is I went brain dead.
The barrel is right at 6 gallons maybe slightly less. I racked from a 6.25 gallon Italian carboy.
As the wine was nearing the top, the realization that it was going to overflow was slightly late. I managed to lose only about a glass. I did have a bottle left to top off for the angels share. It left the barrel quite stained.
Note to self: next time rack a bottle or two off top and use to top off the barrel afterwards.
Thig, the wine was at the end of the 45 day cycle and was clear sitting on its lees. I have heard various ways, but I really want keep the racking and barrel cleaning between wines simple.
I will probably send this wine back through the barrel at some point. Next year in the spring or summer I will have to get another barrel in order to get the wines planned for next year barrel aged.
Looks at this point I will be barrel,carboy aging my wines for about a year.