best way to seal and protect inside sills?

To be honest if you can get at the inner sill in its entirety then I'd give it a coating of clean grease. I've tried various expensive branded rust treatments and whilst they'll slow it down for a while none of them get rid of it for ever. On bits you can regularly inspect and access that isn't a major issue since you can routinely tackle any new corrosion but on the inside of sealed sections it's a problem.

I've used spray in wax where I have to inject into a sealed cavity but if you have open access then a brushing with grease seems to work better than anything else. It will soak into existing pits and deprive the rust of oxygen and it will stick in place but still have the ability to creep if necessary.

The only issue is it is a fire hazard if you're welding anywhere near it - worse than wax treatment.

If you can't get at it in it's entirety then a wax product such as from the likes of Dinitrol or Bilt Hamber is the way to go. Or if you're a cheapskate like me get an oil gun and blast it full of old engine oil. It's kept my Toyota rustbucket on the road far longer than expected.

Messy & cheap but effective.

Literally Waxoil..?

I drilled a series of holes 12" apart down the sill on the inside and fed the nozzle in and 'waggled it about' for 5-10 seconds per hole.

£10 aerosol from Halfrauds did both complete sills with 1/3rd left over.

I distrust using any kind of oil because surely it will just run down and puddle at the bottom of the sill? Hence Waxoil added wax to their oil..

Bilt Hamber Dynax S50. brilliant stuff adn has some scientific testing to back it up.

Used it on my Rocco sills and an endoscope look insuide after a couple of years has me convinced.

www.bilthamber.com/corrosion-protection-and-rust-treatments/dynax-s50

Dinitrol stuff is good too.

If your doing welding you can treat it with somthing like Por15 metalready then prime it with weld though primer and paint any bits you won't be welding too near. Then when it's all welded the cavity wax cam be injected with a normal shultz gun with a flexible extension.

Oil mighy work but it's dirty as hell. The waxes start off very thin so they penetrate really well then go waxy so they don't keep dripping out and collecting grime.

www.bilthamber.co.uk
Dynax S50

Thank you for all the responses, unfortunately I've had some set backs and haven't got to fitting the sills yet.

I do have a question around drainage though. Currently there are a few small slits to allow the sill to drain and they seem to have work as the bottom is fairly decent metal. Would I have an issue if i drilled a couple holes in it so future maintenance is easier? Or would this accelerate rust?