RUST Staining

I have just purchased an 8 year old Sigma ,after giving it a good clean,I fave noticed rust stains coming from behind the trim above the passenger door, running down to the top of the door badly staining the gelcoat, where is it coming from? any ideas?

Hi Bryan.  Had the same problem with my 2007 Sigma.  Started about 2 years ago and gradually got worse.  Every time it rained or after I washed the motorhome the streaking appeared and if left was difficult to remove. I spoke to someone at Auto-Sleeper who assured me there were no metal parts behind the grey trim which could go rusty but suggested that the foam filler was breaking down inside the grey trim.  Did not offer a solution.  I resolved the problem by using Tiger Seal to seal above and below the grey trim and much to my surprise it worked.  The problem has not reappeared after 2 years.  There are other suitable sealers besides Tiger Seal.