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Heat gun and a wider type putty knife........And Patience...
Exactly. Works great, but go slow and if you have access to the back of the panel try to direct most of the heat to the adhesive and bonding strip for the panel that you are trying to remove. Heat softens the adhesive and lowers it's physical properties so that you can gently pry it apart with a putty knife.
A hammer and metal putty knife are typically used. A heat gun can be helpful. You can take a Dremel tool with a cutting disc and run a shallow cut down the center of the bonding joint first to weaken the connection between panels but don’t cut into the underlying bonding strip. A little heat underneath can help get the putty knife started slicing through the bond between the panel and bonding strip. Once started just follow the strip tapping the knife with a mallet as you go. Don’t go overboard on heat as too much of it can soften and distort fiberglass.
This 3M procedure for heavy truck panel repair may be of some interest. Hoods for over the road heavy trucks these days are almost exclusively adhesive bonded assemblies of SMC, FRP, or pDCPD without mechanical fasteners. The procedure below and link describe panel replacement steps when spot or local repair is not possible and the entire panel needs to be replaced.
I used my router with a flat 3/8” bit to remove the black bonding agent just slightly deeper than the panel thickness, touching the bonding strip, but not into it. Then heat and a putty knife is easy. Especially where access to the backside is limited.
