Here’s how I would go about that painting sequence.
1-Prepare the body shell by a lite sanding. I use a wet pc of Tamiya 3,000 sponge or a pc of 3,400 MicroMesh to give the primer something to bite into. Damp wipe the surface with Iso Alcohol and then a wipe with a tack cloth.
2- I’m not a Rattle can user but lite even coats to build up the color coat. Give it 3-5 min between coats. After the final color coat, give the body a full 24 hours to dry and cure.
3- the last issue is the amount of sticky glue on the back of the masks. I always detack even Tamiya tape 1st. You want the least amount of glue as possible to prevent pulling up the paint and or primer. Mask and burnish down the edges.
4- Lite even coats of White till you build up the color intensity. Let the paint dry for 15 min as long as it seems dry, carefully remove the mask. Easy does it.
You never mentioned what primer you used. Tamiya Gray primer in a rattle can is perfect for your application. I even use it for certain small jobs.