I am currently fixing a small hold in the underside edge of the rear sill and was thinking of ways to prevent future rust forming.
I’ve got some rust proof primer, wax oil etc. and was wondering, why not put some of those silica crystals that you get in packages inside the sill to absorb some of the moisture.
Has anyone done this previously? Any reasons it shouldn’t be done?
Thanks in advance 
I think that would overload the crystals very quickly and you’ll get a wet ball of gunk. Personally I’d go for proper drain holes and Dinitrol inside.
Dinitrol (or other brand) cannot get to the non-draining area that rusts. Good ventilation is the answer. Bags of silica crystal (they don’t go soggy) won’t hurt, but they likely need to be retrieved and regenerated fairly frequently. If they are placed inside th sill, that means the trim needs to be regularly dismantled.
Cars stored outside, with a roof in good condition, fair pretty well because the sills are ventilated. Cars in cold, damp garages, can really suffer.
If you are repairing a small hole now, be resigned to the fact that fairly soon you will be repairing another small hole, and another. Rusting is already occurring, from the inside. You can slow the rusting.
Or they’ll go soggy
Sigma-Aldrich (and any others I have used) silica gel crystals do not go soggy. When they are saturated, they turn pink, bu remain crystalline indefinitely. Put them in a low temperature oven to regenerate. Was my standard go to when working with threat agents, and I needed to reduce condensate in the petri dishes. Never went soggy. I wasn’t too fussed with the brand, it might be Sigma, Gibco, VWR etc.
Silica gel crystals in cars don’t do much reducing sill condensate (but won’t do any harm), but are effective at lessening the prospect of mould forming on the hood lining.
Years ago, I flew gliders, and owned one. I stored it at the airfield in its enclosed trailer.
In wintertime, the interior of the trailer would get covered in condensation. Condensation,
over time, is death to the surface gel coat on gliders, so I bought a couple of kilos of silica
crystals from a local chemicals supplier. Poured half of them into 2 or 3 foil baking trays,
and laid the trays at several points inside the trailer, then sealed up the door with
masking tape. Once a week, all through winter, I’d drive to the airfield with the other half of the
silica, lay that down in trays, take the pinked silica home, and oven ready them prior to
the next trip the following week.
It did deal with the condensation, but it was all a bit of a faf.
I think you might find the same, really.
So two small electric fans in each rear wheel arch one on the outside edge and another on the inside edge running off a small power supply would keep them dry over winter if you could stop the fans getting damaged during the summer with road grit.
It is possible to get a direct connection into both sections of the rear sill.
OK it’s getting silly now!!
Or someone invent a clip-on electric blanket for sills.
I think it was Saab and Rover (SD1) who routed engine bay hot air through the sills in an effort to reduce sill rust.
SD1’s were supposed to do that. We all know how well it went
My FE Victor Estate had positive ventilation in the sills via A pillars from the plenum chamber in front of the windscreen.
Alas, it guaranteed wet air passed though them and the A pillars rusted first with the drivers door top hinge welds pulling out of the A-pillar - no signs on the surface, but crusty rotten inside.
No rust anywhere else on it, I really enjoyed driving that car; it handled much better than our Mini and with 2.3litres it went like stink.
Def wouldn’t put silica crystals in the voids. I have six moisture traps on my boat over winter and is staggering the amount of water they trap and deposit in the tanks below and it is very acidic once passed through the crystals.
Awesome thank you all for the responses, sounding like a bad idea going from all of this, was just thinking outside the box!