Hi @galbraithla,
In general, reaction between vinegar and bicarb looks like this:
NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2
Carbon dioxide is gas that disappears from the solution. Water is harmless. And sodium acetate is also known as food additive E262, which gives potato chips a salt and vinegar flavour. It would be very hard to poison anything by this concoction. My bet - sodium acetate will be taken as an additional nutrient with thanks.
It is hard to get rid of fungus on wood because there is always a good chance that part of it is embedded into the wood and protected by it.
What could be done:
- Wood can be scorched with blowtorch. A trick here is to raise temperature inside of the wood to the level when it is instantly deadly to fungus. By the end of the exercise it will be a completely blackened surface. Probably damaged paint on other side.
- Soak wood in concentrated fungicide solution for a while (hours). Painted part represents a problem because it prevents fungicide from entering the wood. Ideally paint should be stripped before soaking.
- If the options above do not look like fun, replacement of the cover is the simplest way to go. Treat it with a fungicide like copper naphthenate before painting, or at least paint it on both sides. Wax dipping is another more bee-friendly option.
In your shoes I might pry off the slats and redo them, or replace with a metal roof remnant if you can get one.