That depends on the material your "tin" can is made from.  Most modern cans are either aluminium or steel.  Best checking system is to hold your can to a magnet.  If it wont stick then it is aluminium,  this will not rust.  I don't know about in the US but here most drinks cans are made from aluminium where as most food cans are made from pressed or rolled steel. 

(My father in law is Professor of Metallurgy he knows about these things!)

I would say that you should have little problems with aluminium oxidising as it is pretty unreactive and has the benefit of light weight. 

Steel food cans are coated on the inside to prevent oxidisation (rusting to you and me) before the canning process starts.  When washed out this shows up as a mattish, often pinky coating.  The outsides are not and therefore will rust if exposed to water and oxygen.

I suggest that you wash and dry the can thoroughly and give it a good blast with a hairdryer at the end to check it is really dry.  Then when sure that there is no water in the can I would coat it with PVA/White Glue.  This is an excellent sealer and will also act as a key when you want to apply the PM to the can.  To be on the safe side I would give at least 2 coats of PVA inside and out before adding the PM and drive the brush into all the crevices.  I would also refrain from diluting the PVA as the less H2O you add the better.  If you wanted to be really sure you have sealed the can properly then a metal paint primer would be best (Hammerite in the UK make most of them).  However you are getting into the region of nasty solvents with these so I would avoid them if possible.

Give it a go.  To be honest the most likely thing to happen to your project if you get some rusting is a red brown stain seeping through to the surface and spoiling your finish.

Let us know how you get on.
Charlotte

Hi Bbee
If you don't mind me asking what are you going to do with these cans and are you covering inside and out.

Sue.

I hope it works for you.  I would be very careful about using them for food storage.  Most of the varnishes that are used for waterproofing aren't really any good for food as the solvents would taint, even dry goods.

It might be worth contacting a local woodworker (or maybe a forum) for someone who turns wood to make containers.  They would be able to suggest to you the safe sealants for food.

I have been thinking along similar lines to you, but in my case I have been thinking about using the plastic cartons that posh ice cream comes in.  The ones I have been saving have a handy lip to anchor PM into  and the plastic interior is safe for dry goods.  I planned to make a rice pot, coffee caddy.

Let us know how you get on

Butterbee, I didn't PM over them, I was using one as a dipper for white glue out of a larger container.  The can started rusting before the glue was dried. I may miss subtle clues, but when I'm hit in the face with something, I tend to see it quite clearly. 

Sue

Sue is has a point there actually,  we should really recycle the steel and aluminium waste that we have.  This is the one recycling process that uses the same amount of energy to use as it does to process the raw material.  (My father in law again! )

It is criminal to think of the quantity of metal human beings have landfilled over the years. 

I will get down off my soapbox now and go back to wielding a drill over my dolls neck joints

Charlotte

Hi Bbee
All sounds good but where's the pic's 


Sue.

p.s  Have you got a gallery space ?