Aw Kevin, you have your work cut out for you!... My recommendation would be wipe down with a spray of alcohol on soft rag. Then your best bet, would be old fashioned Testors model enamel. Use a soft brush, and try and get the longest stretch before loading the brush again. Take your time.
Now, any professional painter will tell you, primer and paint in the same can does not really work. If you decide to spray, Use a primer, primer. Let that dry one full day. Then you final top coat. Let that dry for one full day. Get yourself some blue painters tape, and cut with a small scissors and cover any areas and windows. (This will be the most tedious) However, it is well worth the effort.
And Kevin, you should copy this thread and post it on the car lovers, "Automobiles/ Trucks 1/43-1/50 scale let’s see them"
Plenty of professional car & truck builders. And everyone will love the buses and you renovation........... And plenty of tips The OP has been building and painting plastic trucks and cars, 1/43, 1/48 for 40 years.......... I look forward to his recommendation on painting your bus... 🤩 Either way, I'll be watching'... I love those buses'.. and the ones ten years older'.. when I was a youngster'...🧒🏽
