Hareline Hard as Hull (Penetrator)Hard As Hull is the fly tiers choice for super strong head cements. Hard As Hull is offered in two formulations to best suit every fly tying application. Both formulations feature an applicator brush built into the cap for added convenience. 1 2 oz bottle. Penetrator Penetrates deep into the threads to lock your fly tying materials in place. Won' add bulk or change the surface appearance of your fly.