Anyone have this same rust issue?

[72timemachine]

I have a '72 c10 and noticed what I pray is only surface rust just below the drip rail and above the top of the window's door frame but it does resemble small blisters and I hope the roof is not rotting from the inside out.

When I first bought my truck I removed all the factory seam sealer from the entire drip rail along the top of the cab down to bare shinny metal. I then painted it with this tough spray paint which has glass particles in it to protect it, (which was a pretty impressive product) until I could get back to it and do it correctly. I also applied some exterior adhesive caulk to the area to simulate what the factory did.

Since then I have always covered the cab with a tarp to keep rain water and dirt from going into the cowl area while it has been in storage outside for several years.

Thanks for any help.