Carol, I'm also in Forida.
What you have done is known as a Phoenix graft, attaching a small slender tree to an interesting piece of driftwood.
The main reason for rot on the driftwood is fungi because of the constant humidity from watering your tree daily. Then after fungi sets bacteria may also start working.
Lime sulfur actually play a key role in your case.
Lime sulfur is one of the least toxic, most effective controls for peach leaf curl, brown rot, and other fungus problems. Also used for control of pear blister mite, caneblight and scab. For this it should be diluted at 4 Tbs-1 cup/gal of water.
Lime sulfur was first used on bonsai for that purpose, but then someone realized that applying directly the concentrate on the deadwood not only prevented fungus problems for up to 6 months, but have a desiderable unexpected side effect: it bleached the wood as it has been exposed to the elements (weathered) in nature for years.
Using the lime sulfur concentrate (non diluted) will certainly get rid of all fungus/rot problems and keep the driftwood bleached. In my humble opinion thats all you need. I dont think wood hardener will be neccesary in the future if you apply lime sulfur regularly, say quarterly.
When you do use a small brush or artist brush if neccesary to avoid excessive lime sulfur on the live tree bark and foliage. Cover also the surface of the soil with some plastic (groceries plastic bags from your local supermarket) to avoid excessive dripping of the lime sulfur concentrate to the roots of your tree.
Hope this help you answer the question.
Gene