Painting a Williams by Bachman GP9

My preference it to always remove paint.  Then fresh primer/paint.

Isopropyl Alcohol 91% bath and some light brushing with a tooth brush will work fine.  You may want to soak the model for 24 hours in the bath first.  I've had Bachmann paint come off in 20 minutes and MTH paint in 24 hours or more depending on layers.  Unsure about Williams.

Ron

I would strip the paint as Ron suggests.
I have had success using Tamiya grey primer (aerosol can) goes on easy, provides excellent coverage, great for details and is a good  tooth for whatever paint you decide to use.
I have used it on brass recently and painted with solvent paint. You should check out which plastic friendly Tamiya primer is available.

I am interested in knowing what poly shade colors you you would use?

the solvent paint I used worked out ok but I would prefer to find some non toxic paints in the MEC colors.  
Enjoy the process which ever you choose.

btw, my brass model was undecorated when I started. Had it been painted I would have researched stripping it first.
Ron has provided the best advice for the Williams paint removal.

Question, Pine Tree Green or Harvest Yellow for the GEEPs?

Thank you guys for the response and insights. As to color at least one will be Pine Tree Green with Harvest Yellow stripes