Great DIY!
I've used 303 exclusively on previous verts (Miatas) and use it now on my 135.
It's as good of a product as I've found for this purpose and I don't have a difficult
time controlling the spray (although I understand how you would get even better
control with the BMW repellant). I've always used a brush similar to yours but never
done the vacuuming thing. That's a great addition!