[Hawkman]

Most modellers (from my experience of aircraft modelling) use simple household detergent (usually washing up liquid) in a solution of water. Just make a mixutre where 95% is water, 5% detergent and gently wipe the model with the solution, using a soft rag. You might (if you haven't already done so) spray a protective clear varnish over the model to seal in the paint finish & decals. A matt or semi matt varnish should then give you a good surface for your weathering chalks.

Hawkers