Underbody flush?

I have recently purchased a 1994 convertible. The exterior looks brand new and most people don't beleive it's 10 years old. However, it seems to have a lot of corrosion on the underside. My guess is that since it is a convertible and the manual suggests that you NOT take it thru a carwash, it has a lot of road salt and other road grime built up causing the corrosion. That is since the underbody doesn't get flushed via a carwash.

I guess I have two questions:

-Should I never take it thru a carwash since it's a convertible assuming it would damage the top?

-If I don't do a underbody flush in a carwash, how else can I flush away any accumulated road salts.

I live in the midwest and so road salt is unavoidable.

lift it up on blocks so the under body is high enough to get a very short garden sprinkler that spins or better yet goes back and forth in sweeping motions like a fan. that works well with a well darained floor or out in the country or in a dirt alleyway. i find it works best before i got to bed its on for 5-6 hours. its only necessary once a year. when you have yours cleaned you might want to repair the rusted parts there and then sand it down prime it and when it dries under coat it in thin layers at a time untill its 1/2 a centemeter to 1 centemeter (about half an inch or less) it takes a week or so for good quality undercoat to dry. the inner frame components that have open spaces allowing dirt air and water to enter can be rust inhibited as well to help slow and prevent further decay. after the undercoat and inhibitor set you wont need to worry about it for a long time.
oh yeah stupid me , you can use a car wash just a touchless car wash, machines can be fierce on rag or vinyl tops because they dont have feeling like us humans. some touchless car washes have a underbody cleaner when you pull in at the entrance but thats it then you pull in a bay and use a gun and a soft wash mitt (if you brought one) and thats all you need really but thats me. if you wanted to clean the under body real good drive very slowly over the sprayer.