I've been repairing and doing preventive rust treatment on many of the cars I've driven for over 20 years. Where I live, we get lots of salt and freezing weather, so what I do has proven to work very well, for me anyway.
Instead of undercoating, I've been using a product available at local body shop supply stores called 'flexible seam sealer'. It's made by several companies (3M for one). It has the consistency of peanut butter, can be applied with brush or putty knife, sticks like stink to clean, dry metal and old undercoat, and remains flexible when cured (about an hour). It's the closest I've seen to what Mercedes used. You can paint the metal first if you like, and undercoat spray and/or paint it after too.

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1986 560SL
2002 Toyota Camry
1993 Lexus