Hey Phil,
Once the two pieces of frame are together, I spray a rustproofing compound such as Tectyl or Fisholine, in through one of the sliding hinge holes. I don't go overboard with it though, because depending which you use, it can slosh around and seep out. I tilt the frame around, so that the fluid reaches all areas, then angle frame in such as way as to allow excess to drain out.
If in any doubt that frame may require welding in future, don't even consider rustproofing. I'm lucky in this part of the world, we don't get snow, rarely get frosts, truck lives undercover and always will. I wont be taking truck out in poor weather for a number of reasons, includingpreventing corrosion. That said, it is gonna get cought out in rain some time!
Don't know what types of spray rustproofing you have there, but one of the best penetrating I believe, is Fisholine, a relatively thin compound that gets in seams, crevices etc, and the manufcaturers state it can stop rusting if completely covering the rusted area. I first used this stuff when I was painting classic & vintage cars in 1980s. Back then, Fisholine, as name suggests, had a horrible & almost overpowering fishy smell. Since then, the manufacturers have 'deodorised' the smell away........almost! Fisholine remains fluid consistency for ages, hence the excellent penetration. Tectyl by comparisn, has no offensive odour, is more viscous (spelling?) and forms a skimming surface within hours or days, depending on temp etc. The thicker consistency of Tectyl has created problems when it has been accidentaly sprayed into door window sliding tracks, mechanisms of same, or blocks the drainage channels to doors and such.
I will be spraying Fisholine into the doors of my truck, a bit further down the track. I can virtually guarantee it wont interfere with the window glass or winder mechanism.
