if there is no holes or structural issues with the frame it doesnt need any patching/repair... just clean up the rust spots & repaint with a good quality paint, even rustoleum will work good if prepped & applied right but there are others you can use too. i would call around to some truck/4x4 shops & see what they use/recommend... being up north there has to be some local places that do undercoating etc.

some will suggest POR-15, which is a good product but not designed to be applied over existing painted surfaces so using it on the frame that still has the factory coating intact in areas is not a good idea as it will eventually flake off. POR stands for paint over rust & its literally designed to be applied over clean but rusty metal. fluid film is good at preventing further rust but if sprayed over uncoated/rusted metal it looks terrible & can still allow rust to form since it washes off from normal use. i say paint the frame first then decide if you want to use a product like fluid film.

im in iowa & had some spots of surface rust, no pitting or damage, i wire brushed the areas & cleaned them & the rest of teh coated frame real good, scuffed up the coated areas & used rustoleum brush on paint where i could reach & spray on everywhere else... looks great & is holding up very well after ~2 years. we have just as bad of salted roads as WI & MN.

lots of options to deal with frame rust, just wanted to provide my experience on my 08 4runner.