I have never used this "plastidip stuff so this tip might not be a fix as such bit will pass it along anyways. In the gunstock refinishing bidness some of us "kitchen table gunsmiths" sometimes dabble into, the bane of us is that all the stock finishing com[pounds we use (either commercially bought or our own concoctions) have the same short shelf life as your Plastidip once opened. I think this tip originated in a Brownell's Gunsmithing Quirks volume.
Before you re-cap a half used jar of whatever you are using, add something hard that wont interact with whatever chemical juice you are using to fill the jar/tin back up to as full as you can get it to remove any excess air space and then re-seal. I purposely fill so that some even spills during sealing so I get every bit out. Case in point that this works...I had a small jar of Birchwood Casey stock finishing varnish that has sat on a bench for at least five years since opening the first time and used a bit a couple weeks ago...it was "as new" and Birchwood has a reputation of usually only a couple month's lifespan before it's hard as a rock inside the bottle.