If your fantasy is to have someone take a can opener and peel off the top of your old sedan or if you're contemplating buying a new convertible: Remember, the experts stress, that engineering and strength tests and adequate body reinforcement should be included in the conversion process. Also:

* Never order a conversion solely on the basis of a brochure. "Quality brochures are easy to produce," says Martyn Schorr, editor of Convertibles quarterly magazine. "Quality convertibles are not."

* Conversely, if you can't find some literature on the particular company, that should be a red flag.

* Deal with a reputable dealer.

* During your test drive, put one wheel up on a curb and then open and close the door several times. If there's a rubbing of metal on metal, or the door is out of alignment, the convertible may not be reinforced properly. Floor, windshield posts, doors and frame must be reinforced to prevent torsion and to maintain overall structural stability.

Convertible Concepts Ltd., one of the largest conversion firms, does structural reinforcement and rebuilding work before the original roof is removed from the car.

"The advantage in this," claims chief executive officer Scott Lipton, "is the ability to utilize the alignment and support of the hardtop so the car is never weakened, thereby avoiding any stress to the chassis."

Cost of a conversion? Anywhere from $3,000 to $10,000 and up. Oh, yes, if you happen to have a Rolls-Royce Silver Spirit lying around, it'll cost you $75,000 to have it turned into a proper ragtop.