From the Hylomar Website:
Hylomar® is a polyester polyol–based compound that turns from a gel into flexible putty within minutes of application. Originally developed by Rolls Royce, where it performed under the most stringent aerospace tolerances, Hylomar brings superior qualities to automotive applications. Its non-hardening formula maintains seal integrity even when subjected to thermal distortion and vibration.
This stuff comes either as a blue gel in a tube, a brush-top can, or as a spray-on high-tack sealant. It does not harden, but forms a floating seal between flanges or gaskets. It is great for hard-to seal-areas that will need to be taken apart at a later time. Water necks, distributor flanges, hub carriers and the like. It is impervious to fuel, so works on senders and switches where others will fail and seep. Its the go-to gasket sealant in my shop.
Made in the UK, it is imported by Valco Cincinatti, among others.