Use Styrene it comes in all kinds of thiknesses and widths also can get sheets in different thickness. It's easy to work with, you can cut it with a xacto knife. Score with knife and then bend it at score mark and it breaks. to paint it use a lite sanding 320 grit, paint with real lite coats of paint.
If using Balsa use balsa sanding sealer do several coats and sand in between each coat until smooth with 220 then finish off with 300 sand paper

Hi Steve, If you really want your balsa to look like metal, the best way is to use metal! I mean to use Litho plate, used by some printers, ask around, or try Ebay, I think I got mine from there, you can get in in various thickness i.e. 1mm 2mm 3mm, and it glues onto balsa very easy.

Cheers, Pete

John;
I like to put my two cents on your request.
After trying so many primers, accidently I bumped into one that does the work for me, is not cheap but to me is the best.
Is the best because just with one coat it will cover all of the wood grains, with two coats you will have a super smooth surface whish you can cover with any metalic silver spray.
I bought my at a Discount Auto Parts.

I agree with Admiral Rick. Balsa, especially the soft block - is a real pig to paint to any sort of presentable finish.

I tend to give up and cover with 0.5mm plasticard, with thinned impact adhesive and easily painted as required. This gives a great finish while the balsa supports the thin plastic.

Dr John F Leeds and Bradford. MBC. 😊😊😊

I find that balsa is very difficult to get a really good finish lots of sealing and sanding as the grain seams to show easily.
Rick

If you really want to use balsa then sanding sealer and elbow grease should work. Covering with tissue will also help. I tend to use mirra ply or thin plasticard for small items as the detail can be sharper and much easier to obtain a good finish
dave976