I think before I would try adding a liquid, I'd try dry Sliica powder like that really fine stuff that West Systems sells for thickening epoxy. It used to just be silica used for the different sheens, but I have no idea what they're using these days. I have no idea if this will work or not, but have used that silica thickener for decades. It's so fine that I don't even open the top without breathing protection. Maybe you have some of that in your shop.