the body of my vette started to blister. the car has been painted about 1.5 year ago (blistering showed up already after 4 months). specially the hood shows the blistering. i had sanded the corvette mostly down to the original red paint (had 5 layers of paint on it) as prep for the paintjob

Depending on what you're calling "blisters", there could be several reasons for the paint to "bubble up" on a fiberglass body.
First is usually oils, or some other contaminant in the glass itself. Fiberglass is absorbent, naturally, since it is a porous material. Any oils, or silicons, etc. that settle on the body will eventually get soaked up into the panels. For instance. In A/C cars, the compressor leaks from the front area, where the pulley is. This freon/oil mixture gets slung on the bottom side of the hood. Eventually, this oil will seep it's way to the top side of the hood. So, later on, when the car gets painted, this contaminated area of the hood winds up with oil underneath the paint layer(s). Paint doesn't stick to oily surfaces too well, so it starts to bubble up, or blister.
The best way to solve an issue like this is to clean the bottom side of the hood(or other body panels) with a strong lacquer thinner, and let the panel dry completely, sometimes for weeks.
Sometimes, there are other contaminants that get onto the surface of the body just by airborn pollutants, as in when someone else in the area is using a spray type lubricant such as WD40. Even body oils on your fingers can create paint problems.
Spraying, or otherwise applying a new gelcoat on the painted surface will help, but it won't do much good unless the surface is clean to start with.

I'd be interested in what the paint Rep has to say...

Most good painters I have talked to agree that you should leave the original paint if is solid.  Blistering is quite often caused by just not cleaning the surface well enough just before painting (I say from sad experience).  If they remove the bad paint and properly scuff, clean and seal, I bet you will be fine. 

Be sure to post what the DuPont guy says, although you know he is going to say the product was fine, but the use was wrong.

[QUOTE=art-corvette]...i had heard that best was to sand it down to the bare polyester and than have a new layer of gelcoat on it .... the painter told me that was not necessary and just sanding it down to the original paintlayer would give a solid base to build up the new paint layers...[/QUOTE]

Is it true that having a Corvette media blasted before painting is a good idea?

I was under the impression that they always applied a gel coat to the original fiberglass, then primer, then base coat. The Bowling Green ones got a clearcoat as the final layer.

Art-

to remove the old layers of paint i haven't used chemical strippers... i did it all by hand.. just sanding. The car has been in garage for years so it was not submitted to rain or watever. the underside of the hood was already painted before . At the paintshop they did go through the standard procedures of preparing before painting. i have been checking regularly so i know it wasn't done as a rushjob.