Does a cracked paint blister corrode faster than if grinded?

Question

Recently a paint blister cracked open on the roof of my car. It has rust under it, so my guess is it was one of those lazy ass "repairs", where body filler was added over rust just so it could be painted. With time the rust probably increased until the paint blister cracked open.

I'm tempted to sand the paint blister down to get a better view of the size of the problem, specially considering how close to the rear windshield it is. If it's a superficial rust I could sand it down and apply some primer over it until I can paint it (fingers crossed).

If it's deeper rust, than it could get more complicated, since it's quite close to the rear windshield:

• If it doesn't reach the windshield a small patch could be enough. I'd evaluate if I'm confident enough to do the patch myself, but until then I could add some rust converter and "over-rust" paint to avoid spreading.
• If it reaches the windshield I would have to remove it to fix it properly. At that point I wouldn't be able to apply the rust converter without removing the windshield, and I'd probably have to stop the car at a bodywork repair shop instead of trying to fix it myself.

Whichever case, I'd have to sand it down first to know what to do next. I might not be able to repair it straight away (if it turns out to be a more complicated fix that requires stopping the car at the shop), so I'm wondering if I'll be making the situation worse by sanding it down or if it's better to just remove that blister.

I'm tempted to think sanding it would be better (as long as I at least throw some "over-rust" paint after) because those cracks could be holding water inside and making the corrosion faster?

Answer 1

The blister in the paint holds moisture inside accelerating the rust. A practical repair could last a year or two is a grind to remove most rust, conversion ( phosphate ) treatment then prime and top coat or just topcoat. A serious repair that would last longer and match the paint better would be expensive. Once rust has started it is difficult/expensive to stop. There are many variations of a "practical" repair ; such as grind most rust, conversion coat, fill/cover holes with polyester/fiberglas mat or epoxy putty , grind , body putty, sand, prime ,topcoat. I have done them them all but one to two years is the life of the repair. I have had to junk a couple good running cars because of rust ( salted streets in the Chicago area).