Dss902,
I build a lot of Tamia bikes - gotta say I've never sprayed the tires. I just hit them with 320 wey or dry sandpaper (dry) in the direction if spin. Works especially well with slicks.
I've used Tamiya flat base on numerous occasions - and found it to be harder than necessary. Just use Humbrol Matt Cote if you need a flat base. They also have a Satin Cote and a Gloss Cote , but I prefer Future for a gloss cote
The Future is perfect for the job. Just make sure you allow any water based acrylic paints to dry completely before decalling and clearcoating. I've had the paint shrink after a while, cracking the decals and clearcoat.
Cheers
LeeTree