The B&G SpotSprayR Sprayer (#24000107) is 16 oz.
Yes, the top of the B&G SpotSprayR Sprayer (#24000107) is plastic.
We are not aware of any extension tips for the B&G SpotSprayR Sprayer (#24000107). We recommend contacting B&G and asking if the wood hardener can be successfully used in this type of unit as we are not familiar with the chemical. (800) 544-8811
According to the manufacturer, the tops of the B&G SpotSprayR Sprayer (#24000107) and the B&G QT-1 Handheld Sprayer with Adjustable Tip (#17017401) are the same.
The B&G SpotR Sprayer comes with an adjustable nozzle that can range from a pin to a cone spray pattern. The pin stream would be used for crack and crevice applications. If you need a more narrow stream or tip, you would have to use a different device as there is not a C&C tip available with a straw on this unit. You may find an aerosol application will work better for you in this scenario and use the crack and crevice straws for direct void treatments.
The B&G 4 in. Wood Injection Tip (22071827) can be used with B&G pump sprayers only but requires Tip Cap Part 1325 (sold separately). It would not work with the B&G SpotSprayR Sprayer (#24000107). The B&G 1" Wood Injection Tip Whit-1 (22071824) could be used but only with the B&G AccuSpray with Termite Injection Tip as a replacment.